RMC discap voltage rating

[WB2CAU]

RMC disc capacitors were as common as beach sand in the 1960s.  I have some stashed away in my junk box but although some values have a 1 KV rating stamped on them, others have no rating marked at all.  One example has stamped RMC, 22NPO, 10%.  So it has the brand, the value, temperature coefficient, tolerance, but no other coding that might suggest a maximum voltage.

How can I determine voltage rating?  I haven't been able to find any RMC data online or in my archives. 

I've applied as much as 770V (maximum voltage from my Sprague TO-6A) to the sample described above with no indication of failure but I'd still like to know what the published rating is. 

Any RMC discap experts on AMFone?

Thanks,
Eric