I know that this thread is getting a little stale, but thank you for posting your pictures. I just purchased a USGI tin of M1carbine ammo and realized that I needed to jump back onto this subject.. So it looks like there are at least four examples of this same buttplate on Detroit M1 carbines. It would be nice to know who made the buttplate and stock. I also see that the butt area under the magazine pouch is a lighter color just like mine.
The history around these Detroit M1's is just fascinating. They have been kept under governmental control for almost their entire life with apparently a minimum of tinkering. All of your pictures are so similar to mine. I'm going to craft an email to Zander to see if they can tell us how many they received from Detroit and any other details. It would be neat to get a registry going of these rifles considering their unique nature.