i would have you measure the base and compare to known calibers.
They are lead so treat them accordingly . Lead and slightly alloyed lead is wanted so that it upsets enough to engage the rifling. Yes today copper is making inroads. Expect alloys mixed in too.
I'd soak in simple water with a drop or two of dish soap. Them once softened very gently coax dirt out. A paint brush better than a toothbrush for such soft material.
Lead is soft material so gouging or scratching is very easy.
Hollow bases /skirts don't tolerate real high pressure.
There are calibers and bullets that want gas checks on thier base. That's like a cupped copper plate to protect the lead .( Often seated behind a flat based lead bullet.)
Lower pressure black powder and reasonably light loads of it is where skirts became popular. (See Minie bullet /ball.)
https://en.wikipedia.org/wiki/Minié_ball
If the base doesn't want to clean out don't fight it. Could be someone wanted it that way for some reason.
A too stout charge of powder could damage the skirt and allow pressure to blow by.
i know it doesn't answer your question of age but by getting dimensions you can compare to known calibers and designs.