The reason Shield needed to invent their own footprint is because Shield sights are remarkably compact. This footprint is designed in a similar way as the Noblex/Docter footprint (there being two holes for the screws and four sockets for the notches on the mount/adapter, one in every corner). The sockets, though, are a tad broader and closer together with each other.
Holosun 407K and Holosun 507K feature a modified Shield footprint. The dissimilarities are:
As a result, it is a requirement for some pistols that are optic ready for Shield footprint to be modified for the 407 and 507K. That is, the two rear indexing lugs on the pistol need to be taken away and the two front indexing lugs reduced in height. Instances of these pistols are Hellcat and Walther PPS. However, some pistols, like Sig Sauer P365XL, do not need to be modified.