18mm cartridge found in the back yard - what is it?

This turned up in the back yard and appears to be quite old. It has no markings

18mm x 38mm

Hell it may not even be a cartridge but a plumbing item or some such curio, but I would like to know!!

Thankyou!
IMG_20191017_094017164_HDR|690x388

Welcome to the Forum!

Nickel plated brass, odd “rim” and rusted steel object in the “primer pocket” (?).

Does the rusted steel object extend into the “case”?

Hmmmmm! I don’t know!

The only thing cartridge wise remotely close that I can find is the French 18x35 R Tabatière Rifle Mod. 1867 cartridge but the case wall is paper/cardboard. All this means is I still don’t know!!!
http://www.municion.org/tabatiere/18x35.htm

Brian