I don’t know beans about these, but I find the “X” above the model/serial # to be extraordinary. The surface is concave, yet when you look closely, the mark wasn’t made with a flat die stamp, the die was made convex to match. any chance that’s meant to indicate a roman numeral 10? (for a repair/overhaul depot?) not questioning anything as to origins & who did what, just pointing out that it isn’t an ordinary rc ‘x’, it has serifs, not made w/4strokes of a cold chisel, crudely.