Ronson fuel canisters are 2.75 ounces of iso-butane – might be a good size for lightweight backpacking
After years, I finally emptied a canister so I tried removing that plastic piece:

Bottom is a working canister.
Lid to left.
I tried removing the plastic piece from top canister. It deformed rim a little, then the plastic piece with spring pulled out. There was a tiny bit of butane left which immediately dissipated.
If you wanted to use this with a stove, which would have to be Roger stove, maybe you could just cut off the plastic piece to a point where it would work?

