looks a decent cab, but that white stain would bug me. yeah the back panels were a better grade back then, made from mdf, not chipboard. they seem to use chipboard instead thesedays. there is a difference.
mdf = medium density fibreboard (dense panels made from fine wood grains or 'sawdust')
chipboard/particle board = (less dense and made from wood chippings)
the chipboard on modern cabs is all over the front of the baffle too where you cant normally see it, hidden by the grill material. £600 GBP retail for cabs that are half made from chipboard and loaded with G12T-75's. people still buy em over much better made cabs from other brands, or older better made marshalls for half the price.
pretty sure the JCM800 cab baffles were still made from ply & pine all over, just less of it than the 60's cabs.