The magazine latch mechanism on Duramatics. Engages a rectangular cutout in the left side of the magazine, as shown in the photo below.
All H-S mags from the mid 50's on, had this rectangular cutout. Except the military extended base magazines. As far as I know? Most repos like Triple K's, also have the cutout.
After the Dura-Matic was introduced, all High Standard magazines had the cutout for the Dura-Matic model. Some extant old stock was even reworked by milling the pocket. Both the slant grip magazines and the military grip magazines had the cut out since both style magazines used the same inner and outer shell stampings.