Your images show a perfect combo. The 5 likely means 5" barrel. Inexplicable is that all Clark's have model numbers in the company's catalogs but these numbers are never marked on their holsters; instead only a number for barrel length and sometimes for frame size.
The 999 but on page 185? Only similar by being a Clark and a crossdraw; otherwise they are constructed very differently. Clark used the 999 number on three
different holsters in Clark's lifetime including a shoulder holster. Clark Holster operated until 1959 when it was superseded by Bucheimer-Clark that a decade later was swallowed up by Tandy.
Yours, Clark called the Combat and it was copied by every maker by the 1960s except Bianchi and Safariland. Hume, Safety Speed, everybody. This is 1938:
1938 stamp (3).jpg