i think you're correct, the morecombe units were the test-bed for AC traction, albeit at 6.25kV?
apologies michael, the way i read the info was that the MSJA units had been converted - re-reading, it seems the pantos were changed to SF type to match the new catenary installed as a precursor to the line and infrastructure being made 25kV - the units were still 1500v DC
incidentally, which of these units (if any) were converted from former watford stock? i'm a bit confused with this, as some o/h units clearly have bogies with shoe-beam brackets visible
as an aside, is this the only time SF pantos have been used on DC on BR?