That's evidently what they called it at SA, a boxm. Hard to pin down the limits of the open box rebuild stamp, but best guesses are in the early 1950's, maybe 1951 to about 1954. The M1Cs rebuilt for the Korean War had them and they were done in that period. Later SA rebuilds show a DAS. Collectors think the DAS at SA was used in late 1953 or early 1954, but we are guessing.