good one, Bruce.
That stuff does indeed seem to rub away easily once it ages. I thought of clear coating the numbers I had but not knowing what the ink/paint was, decided against trying. I should have put all the RCU’s I had under my Black Light back then to see what that revealed.
In fairness though, I doubt any of the designers or manufacturers expected them to last 80 years and if I think about it, these items in particular do not seem to hold up well in damp conditions. The wooden boxes just fall apart.
David