The 2 inch model (CRU) isn't much different in design from the 1 inch CRM:
https://stevenjohnson.com/oscillosco...tional-cru.pdf
But i'm guessing that the 3 inch model might use somewhat higher voltage. The max for the 3 inch CRT is specified as 1500 Volts, so C2 might have to be rated at a higher voltage.
jr