I looked at the springs that hold the lid in place and everything looked fine
UNTIL I held the 503 lid next to the 500 lid and realized that a spring on the 500 is broken. It's such a smooth, square break that it looked like it should be that way.
I cannibalized the spring from the 503 and the lid now stays up. I wish I could swap the whole upper plate, complete with hinged lid but the 500 and 503 top plates are not interchangeable.
I still haven't threaded it and sewn anything, maybe this evening.
UPDATE: It Sews!