I have a B25 from 1975 -- upon inspection from quite a few people, it seems it's on it's original finish. Glossy. Hard as nail. Almost no marks on it. Can't be just oil...
Now I've seen a recent browning (a year old) with bumps, dents, marks and so on right into the wood. Sure, some of them might 'lift up' with steam, but what amazes me is that my B25 is nearly pristine, the finish is more or less bulletproof, part from a bit of wear at the top.
So, what finish did they use? it's not oil, it's not 'varnish' -- it's not acrylic, and it's not nitrocellulose (as it would have obvious signs, like my guitars).