That transformer may be intended to drive what's called a 70 volt system. That's how many "distributed" PA systems are wired, like those with lots of speakers in the ceiling. Like in an office, or a department store. Possibly your projector came from a drive-in movie theater? Lots & lots of speakers in that situation. There's a little transformer on every speaker that takes the high voltage line down to a level where it can safely drive an ordinary speaker voice coil. This kind of system allows very long speaker runs done with skinny wires. Imagine how much copper it would take to run 8 ohm speakers directly at distances of hundreds of feet.
In any case, it looks like that OT would be unsuitable for driving any ordinary speaker load we encounter in guitar amps.