Tom is correct. The first test magazines were 20 rounders. Furthermore I used to see many "Fantasy" markings on magazines because a standard magazine was 50-75 cents, but if you convinced somebody you had something special, it would sell for $5.00 and up.
I have seen Army Air Corps magazines, Rock Island magazines, WWII Dated magazines with 1960's FSN's and a lot of nonsense. I checked and found no US Co as a contractor.
The follower can not be a hold open. The T18 follower was designed and tested by Autoyre and not adopted until April of 1945. Autoyre made 250 of them and tested them in the 15 round magazine.
Can you post a picture of the follower?
Sorry but none of the markings on this magazine make any sense.