Anyone seen this on back of Receiver?

[W5USMC]

Question for those knowing more about these SG receivers than me, how come mine has a narrow tang?

Bob, there were 5 different receiver configurations produced by Saginaw for inland, three of which were narrow tang, I believe it was their later two configurations that were wide tang. Their marking also changed throughout production SI, large SG, small SG.

CCNL-346