There probably was a few go out the back door towards the end.
Thing is, I was able to check under the buttplates of all I mentioned, except the two M-10 .280s
They all had proper serial numbers stamped in both the steel and the wood. This indicates to me that they went out the front door. Most likely with serial numbers on their barrels originally.
I`d be inclined to think that most of the odd configurations, were more likely rifles put together as prototypes or to float ideas.