Ah, so that's what those are.. I have some nos somewhere.
If pushing doesn't get it in, it may need a bit filed off the seam edges so it can close a HAIR more.
It would likely need a thin points file (contact point files are very thin. An automotive version for old school ignition distributors is what I use for a thin file.)
A good diamond dust metal fingernail file is an alternative. (thicker though)
I also see a slot. That doesn't fit over something does it? If so, is it aligned?