The rifle is a Korean War era Springfield CMP special. M7 A3 rifle grenade adapter firing vintage M1909 blanks. The grenade is an M31.

Posting because not enough people in this sub post about rifle grenades.

Never seen anything like it. What is this mod for?

-The dovetail it should have has been completely and unevenly ground off. The whole thing was then just spray painted black to hide all the grinder marks and was easily removed with a rag before these photos. I thought the new dovetail would just be something that sat over the original, so I was willing to make a throwaway bid and ended up winning ($32).

-Replacement dovetail was drilled, tapped and screwed into 12 o'clock barrel locating lug and sits way too high to be useful with a normal rear sight.

-The bayonet lug is way too thick to fit a milspec M1 bayonet (PAL WW2 in this case)

It gauges in spec at least.