There is no such thing as a Milner 212. 212 was included on the plate because that is the boiling point of water. The idea being that in a fire the heat had to boil off all the water from the sawdust and alum crystals in the “fire resisting” lining before the contents could reach a higher temperature.
Any competent locksmith should be able to open that by picking. I can’t see how deep that hole is but the person drilling it probably got as far as the hardened plate and couldn’t get any deeper. If you know the drilling point, of course, opening it is as easy as can be, once you have got the hole made. With modern techniques that is very simple in itself. That unit was designed to resist the old burglars and offers almost no security against modern attack.
As it is over a century old the fire resisting lining is likely to have settled down so the top part of the safe will have spaces in the lining rendering it worthless for the purpose of fire protection.
I hope you have fun opening it etc. They can give hours of pleasure but they are virtually valueless.
