Well, I stripped down the PSU and the Flyback is a basic transformer with the secondary grounded and one primary, no extra primaries for oscillation feedback.
Secondary is open @ 20MΩ. Took it out of circuit and tried for a 2mm spark using a DC supply to 24V and tapping the primary ( 0.25 Ω DC, 1.4Ω impedance @ 1Khz) on/off manually with a heavy wire feed. Got no sparking at all.
Two IRFP450LCs drive the primary. They seem ok but look like refurbished 'pulls'. Some rework was done before on these FETs, solder balls around & flux.
Ordered a replacement flyback and replacement FETs as well.
This seems like a good way to load test a running HV system.
**broken link removed**