Well, a quick search for the transformer part number seems to indicate that it is a proper Sound City transformer, probably by Partridge - though this only comes from one hit on an eBay seller in Spain. The part number is T.G. 9875-A265.
Here's the front of the amp chassis (some valves removed at the time):
And the back. The PI valve sits between the two EL34s. This is a later arrangement as the PI was originally the other side of the metal shielding plate with the other pre-amp valves:
Here's the only really suspect cap I could see with obvious cracking (though no leakage):
Here's the bottom of the two main smoothing caps. They both seem to have a small bubble in the rubber/plastic covering, so I'm not sure if that's normal or not.
Haven't had time to do any more real investigation as I need to tidy away all the stuff from the 'indoors workshop' (i.e. the dining room table), as we have friends coming over for a meal tomorrow.