Hi, I have an old crystal unit quite large about the size of an
average octal valve, metal circular,

Ron-

Does the cover come off? Are there screws holding it onto the base? If it
isn't a hermetically sealed unit, it might be a crystal oven with a heating
element and either a thermostat or proportional temperature control. Such an
oven might have just about any frequency crystal inside.

If the unit is hermetically sealed, it may not be an oven, but could be a
temperature-compensated precision crystal, perhaps something like 100 KHz or 10
MHz.

The 10/X C/2 doesn't ring a bell to me. I would examine the surface more
closely and see if there is any other text painted, stamped or engraved
anywhere on its surface.

73, Fred, K4DII