I'm guessing my cleaning caused the colour loss to the interior of this piece near the rim

But I'm curious as to what it actually is. The pieces that came off looked as though they could be a thin plastic coating. It also reminded me of nail varnish - the bits that flake off, except that the pieces curled up.
Any ideas what it could be, and how it was applied? Is it just enamel? I just didn't expect it to come off

. I was told it dated back to the 1950's(ish) or thereabouts, so would this tie in with the way in which colour was applied?
TIA
