So, progress report.
I've cleaned and oiled; dis-assembling the tension unit (which is weird) and the shuttle race but nothing else. It's not in bad condition and I didn't need to take anything else apart to release stuck mechanisms or whatever. As it's more complicated that a hand-crank Singer, and I have no manual I decided discretion was the better part of valor. If it moved easily, I left it intact
I've threaded it, and test stitched a straight line, and... it sews!
Tension seems sweet even though I've had it apart, stitch length does vary as it should, although the numbers on the dial had me confused. They go from 6 through 8, 13, 25, right on to 55. But 6 is the longest?
We think the numbers must be stitch per inch or similar, because it's definitely 55 is the tiniest, and 6 the longest.
The settings were full pressure on the foot pressure, full height on the feed dogs, and 'M' on the B.A.M. slider.
Now I'll change feet on it and start with zig-zag and see how it goes, but I was excited to share, and this info might prove a useful resource eventually to someone else with a machine like this.
P.S. It might be a male, this machine. And I might call him Valor. What do you think? It's certainly the heaviest machine I own.