I ordered my OTS 50 without Trannies in order to reduce shipping weight and thereby reducing shipping costs.
Instead I used Hammond PT and choke. For OT I used a Mercury Magnetics that previously had ordered for a 59 Bassman Ri combo. I am satisfied with the sound, but can obviously not comment on how they compare to Nik's transformers.
For my latest build - a Dual High Plate/HRM - which I have sourced myself, I have used a set of old Partridge Tranformers from an old 70 Laney amp. I have 580volts on the two EL34 output tubes and it probably delivers 80 watts. Its way superior to my 50 watt ceriatone.
I hope my reply might inspire you even if it does not really answer your initial question.