Acquired a pair of L300's, these have 3 caps that are wax filled, wrapped with a cardboardy-paper.

I saw the thread where someone rebuilt these, and other people have just replaced them.

But what kind of cap is it inside there? Film?

I know JBL in this era used metal NP's on some models, yellow mylars on lots of models, L300 included, but if the wax caps are film, then they should still spec fairly well, like the mylars do.