Supposedly the grips with 13 rows of checkering between the diamonds are Remington-UMC grips, but there are just too many pairs of 13 row grips around to all be Remington-UMC. There were 25,767 Springfield 1911's built, and only 21,676 Remington-UMC's, but the loose 13 row grips turn up on a regular basis while you never see the 11 row Springfield grips.
I suspect that they may be contract replacement grips due to the number seen. Shown below is a set of grips that came off a butchered 1911, with the bottom being 13 row and the top 15 row.
Wow! You guys are REALLY into the details on these things! I'd just be schtickled to have an old Colt period. Any mixmasters out there anybody wants to unload? (Kidding! This is NOT an offer to buy!!! I know the rules here.)
You guys really need to invest in Charles Clawson's Collectors Guide to Colt .45 Service Pistols. It'll tell you all of these things, making it easier to know what you're getting yourself into when you go shopping for a USGI .45 Auto.