47mm ''Horseshoe'' i.d

[Mag]

Hello Gents
Picked this up some time ago in Italy sold as trench art , 46.65mm at its widest point (missing driving band) , 173mm long,base threaded and im assuming a missing ballistic cap but what puzzles me is the perfect horseshoe cut out of the side , its machined and not a gas axe cut.

Anyone have any pointers as to its purpose ?

 

[Falcon]

That would be a complex shape to machine. Someone went to a lot of effort to do that, which to me makes trench art seem unlikely (to me anyway).