Voltage tolerance of K75-10 Russian caps.

Hi all,
I am using some K75-10 caps as coupling between stages in my SET amp. The spec printed on the cap is 250V @ 50Hz. The caps run with 230V across them when the amp has settled down, with around 410V peak. However, on startup the PD reaches 505V for a second or so.

I lknow that these things are fairly bombproof, but I don't want nasty surprises, so am I risking some magic smoke or is this an acceptable pulse voltage?