My understanding of the definition of "NRA 100%" is new, never owned, never sold. So a 1968 gun with no box and no papers cannot possibly be 100%, because it has been bought and sold at least once, right? Well I guess it is theoretically possible that a 1968 gun is still in the shop where it was distributed to, unsold. But I would think it more likely that the sun explode today.
I think you are looking at a possibly 98% pistol.
Disclaimer: I got all this information off the internet, so it could be total bull.
IMHO that's a bit steep, but not outrageously so. If it still had the box and manual it'd be worth the price. Since those are missing it's more a matter of how "perfect" the pistol itself really is. It was around the mid-1960's that Colt first began having some issues with quality control, once the company had been bought out by a larger corporation and the workforce unionized. A lot of the old hands that had been working there for decades either retired, or left after being dissatisfied with the changes going on. The result was a lot of "new blood" was brought on board, and their lack of experience with shaping, polishing, and fitting the parts showed. For example, my father has a 1966 Government with a badly mis-shapen triggerguard and uneven slide edge, which was something I never saw on any Colt made prior to that time period. The majority of those I have seen since have had some sort of flaw as well. If the example you see displays no apparent cosmetic issues then it was likely put together by the workers who knew what they were doing, in which case I for one would be more inclined to pay a premium for it.