mapj1:
By mains transformer and rectification - no need for clever inverters when 50Hz is available. The voltage is not that inconvenient, I'd expect a voltage multiplier rectifier to keep the winding efforts reasonable - Cockroft and Walton style. Maybe not as big as the ones in the photos on that page.
MHRestorations:
I have a V.154 and can confirm the flash test voltage is derived from a small transformer. It's actually AC coming out of the probe. I do still use the flash test on equipment that's been repaired/modified. Absolutely lovely bit of kit
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