[CambridgeWorks]

I remember the aluminium can transformers used as mod or ht inverter back on early gear, even W30AM, but thought they were resin filled as the bottom was just crimped over.
I think you are on the right track regarding secondaries though.
ISTR a compressor winding feeding back to the earlier stages and a public address one as well being included back then, which would be an option in the internal wiring and control box. It was easier to standardise by putting the PA winding in place. Whether yours is, I don't know. The compression one would be tens of ohms, the PA very low resistance.
I well remember in the late 70s pulling up outside my mates house to pick him up on a Saturday morning and he wasn't up. I selected PA option on my Westminster W30AM (there was a 5 or 6 inch horn under my transit bonnet). I then played the "Time" extract ftom Dark Side of yhe Moon, with all the alarm clocks going off. A few curtains nearby were pulled back to see what was happening!
Rob