Checked the Blue Book and they do list 9mm for the SP101 but no mention of US marked.
...Standard Catalog of Smith & Wesson: 3rd Edition, the 2" Model 56 (USAF exclusive) was marked "U.S.," on the smooth back strap and the 4" Models 15-1, -2 and -3 were marked "U.S.A.F." on the left side of the frame. I see no mention of markings for the 2" USAF Model 15's.do you remember how it was marked? I know the 2" was issued, but they are quite rare today/