Here is the long, drawn-out answer to your question, copied from an old post I made in the Colt FAQ thread:
Series 70 vs. Series 80
The term "Series 70" originally referred to the spring-collet "Accurizor" barrel bushing and matching reverse-tapered barrel used in Government Model and Gold Cup pistols starting in 1970. Prior pistols became known as "pre-Series 70" guns, and since the collet bushing was not used in the Commander models there were technically never any Series 70 Commanders. The collet bushing was finally phased out around 1988 or so, several years into Series 80 production.
The aftermarket began using the terms "Series 70" and "Series 80" to identify parts they offered for pistols with or without a firing pin safety, such as extractors and firing pins. Eventually the use of "Series 70" to identify any 1911 without a firing pin safety caught on and even Colt began using it in that manner as well, calling their Series 70 reproduction, Wiley Clapp models and various WW1/WW2 replicas by that name even though none of them use a collet bushing.
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Historical background:
Colt is the original manufacturer of 1911 pattern pistols, having made versions for both the military as well as commercial market since regular production began in January 1912. The commercial versions were nearly identical to the military ones, differing only in markings and finish. Following World War Two military production ended, but the commercial guns remained in production with only minor changes such as deletion of the lanyard loop and a larger thumb safety shelf. These pistols are known to collectors as "pre-Series 70" guns, as they pre-dated the Series 70 guns introduced in 1970. It was during this year that Colt introduced the first major design change to the Government Model in nearly 50 years. In an attempt to improve the accuracy of production guns the barrel bushing was redesigned, along with the barrel. In this system the bushing utilized four spring-steel "fingers" that gripped the enlarged diameter of the muzzle end of the barrel as the gun returned to battery. By tightening the fit of barrel and bushing in this manner Colt was able to improve the accuracy of the average production gun, without going through the expense of hand fitting the older solid barrel bushing to the barrel and slide. Models using the new barrel/bushing setup were the Government Model and Gold Cup, which were designated the "Mark IV Series 70" or simply Series 70 pistols. It should be noted that the shorter 4 1/4" barreled Commander pistols retained the use of the older solid bushing design and thus were never designated Series 70 pistols, although one hears the term erroneously applied to Commanders from time to time. The new "collet" bushing (as it came to be known) generally worked quite well, however it was occasionally prone to breakage (see post #3 below) so it was eventually phased out around 1988 as Colt reverted back to using the solid bushing in all of their pistols.
The single biggest change to the 1911 design came about in 1983, when Colt introduced the "MK IV Series 80" pistols. These guns incorporated a new firing pin block safety system, where a series of internal levers and a plunger positively blocked the firing pin from moving until the trigger was pressed, thus eliminating the possibility of the gun discharging if dropped onto a hard surface or struck hard. In this instance however, ALL of Colt's 1911-pattern pistols incorporated the new design change so even the Commander and Officer's ACP pistols became known as Series 80 guns. With the previous paragraph in mind, it is important to know that from 1983 until 1988 the early Government Model and Gold Cup Series 80 pistols used the Series 70-type barrel and bushing as well, although they were known only as Series 80 guns.
There was one other design change made to the Series 80 guns as well, and that was a re-designed half-cock notch. On all models the notch was changed to a flat shelf instead of a hook, and it is located where half-cock is engaged just as the hammer begins to be pulled back. This way the half-cock notch will still perform its job of arresting the hammer fall should your thumb slip while manually cocking the pistol, yet there is no longer a hook to possibly break and allow the hammer to fall anyway. With the notch now located near the at-rest position, you can pull the trigger on a Series 80 while at half-cock and the hammer WILL fall. However, since it was already near the at-rest position the hammer movement isn't sufficient to impact the firing pin with any amount of force.
Regarding the "clone" guns (1911-pattern pistols made by manufacturers other than Colt), so far Para-Ordnance, SIG, Auto Ordnance, Remington, and Taurus have adopted Colt's Series 80 or a similar firing pin block system as well. Kimber's Series II pistols and most models of S&W 1911s have a FP safety also, but it is a different system than Colt's and is disabled by depressing the grip safety. No manufacturers aside from Colt ever adopted the Series 70 collet bushing/barrel arrangement, so technically there are no "Series 70" clone guns. What this means is that design-wise most of them share commonality with the pre-Series 70 guns, using neither the firing pin block NOR the collet bushing. Because of this it is important to remember that only Colt Series 80 models, and a couple of "clone" 1911 makers use a firing pin block. Older Colts and most other clone guns lack a firing pin safety and can possibly discharge if there is a round in the chamber and the gun is dropped on a hard surface, or if struck a blow hard enough to allow the firing pin to jump forward and impact the primer of the loaded round. Typically, use of an extra-power firing pin return spring and/or a titanium firing pin will significantly improve safety in these older designs. By the way, for the past decade Colt has been producing new pistols out of their Custom Shop that lack the S80 firing pin safety. These are the Gunsite and Wiley Clapp models, WW1 and WW2 GI replicas, and a reintroduced original-style Series 70 in both blued and stainless steel that should appeal to 1911 purists. Interestingly, all of these use a solid barrel bushing, so mechanically they are more similar to the original pre-Series 70 models despite being advertised by Colt as having a "Series 70 firing system".
Regarding the controversy involving getting a decent trigger pull on a Series 80 gun, it is only of importance if the gunsmith attempts to create a super-light pull (under four pounds) for target or competition use. In defense/carry guns where a four-pound or heavier pull is necessary, the added friction of the Series 80 parts adds little or nothing to the pull weight or feel. A good gunsmith can do an excellent trigger job on a Series 80 and still leave all the safety parts in place, although he will probably charge a little more than if the gun were a Series 70 since there are more parts to work with. But any gunsmith who tells you that you can't get a good trigger on a Series 80 without removing the safety parts is likely either lazy or incompetent.
They can be made to work very well.
