I would guess this is a primer.
It is an old british patent by Bailey.
A shotshell primer.
I am sure others can give more info.
Yes Rene is correct, it is a primer manufactured to Bailey’s Patent.
In the early days of cartridges the cap flashed back round the sides of the priming cup and cap (they were not a good tight fit) and as it was harmful it quickly eroded the breech face of the action so Bailey invented a cap that had a very thin brass foil wrapped over the top of the cap and the cup to protect the gun from this attack. I believe he had another variation were the complete base of the cartridge was covered too (no examples of that, sorry).
It was soon out-dated as better fitting caps and cups, plus different compositions of the compound in the caps.
I have attached a picture of a loaded cartridge by Joyce who used Bailey Patent (without checking I believe only Joyce took up his patent, could be wrong on that point).
Some more bits from Jim reference the Bailey’s
You can see the imprint of the cap quite clearly in the left hand one and the right hand it with the cover all over the base.
Explanation of the function.
Bruce, here are excerpts from Bayley’s 1882 and 1884 patents showing both primer constructions: