@carbonbased
This trigger housing has nothing to do with a registered, drop-down, push-pin receiver. The holes you see are non functional.
Clip-on-type trigger housings are designed to work with a Title 1 semi-auto receiver like those found on the HK 94/SP 89 firearms. The clip-on ledge was a feature of the HK semi-auto receivers designed to prevent them from integrating with the HK push-pin type of machine gun trigger pack. It prevented the easy conversion of a semi to a full auto weapon.
Several domestic manufacturers became adept at modifying trigger packs to incorporate an auto sear and lever into a semi-auto trigger pack that mounts to a clip-on type of receiver. The other option was to modify a push-pin machine gun trigger pack to mount in a clip-on receiver. This was not easy. Converted packs using a registered sear had sear and pivot pin mounting holes in slightly different locations which affected the cyclic rate of fire.
These conversions were not as desirable as registered receiver conversions that could use standard push-pin types of trigger packs.
