Also, the letters 'P" for proof mark will be on the frame, just below, FJA for Frank J Atwood, and the P on top of the slide just forward of the rear sight. If it is a matching slide and frame gun, these "P"s on the frame and slide will be the identical strike and font. Also, if the slide and frame do not mate up even where the hammer is, then the slide and frame are not correct.
Most correct and all original 1911A1's were brought back by GI’s from the War.
Guns that were released from the DCM were all re-built. Rock Island, Augusta Arsenal, and maybe Springfield Armory (Mass) would take a mess of these guns, take them apart and put the parts in bins. They would then take the parts, inspect them, make sure they were serviceable and assemble guns.
Lots of guys out there will advertise an all original 1911A1. Sometimes they don't know what they are talking about sometimes they are misrepresenting an arsenal rebuilt gun as 100% original.
The worst fellows are the ones who have a cobbled together Remington Rand Slide and aftermarket Essex Frame, that try and pass it off as genuine. I’ve been to at least a half dozen gun shows where they are asking $800.00 for something like that. I think Bruce Canfield has a book on the 1911A1, might be wise to get one prior to plunking down good money.