50 BMG Subcaliber case or just the beginning of trench art?

Hello all,

I obtained a 50 BMG case at a flea market. It is headstamped CAL 50 FA 35, and is lacquered brass. The hole in the head is 0.42" diameter. It fits a 38 special case fairly tightly, but the rim of the 38 special does not fully seat in the hole. The inside of the case appears to have quite a bit of powder residue in it. Below is a picture of the head of the case. I’d like to know if this case was actually some kind of special sub-caliber device or blank device, or if it was just in the process of becoming trench-art. Thanks.

Beginning of “trench art”, or a dingbat.

Quite possibly some sort of alteration unrelated to use as a cartridge.
Plumbing adapter to step down to a small size piece of pipe or tubing; replacement bearing in some machinery; hole in the base large enough to stick on a nail or bolt on a fence for decoration, etc, etc.