......in the bottom of an ultrasonic jar.

But I have no idea what it is. I remember cleaning a bunch of general movement parts so it could have been then.
It sort of rings a bell, maybe something Seiko?

Any ideas?

 

Bezel retention or click spring upon first glance.

 

It is a Seiko movement holder spring.
Take a look at the movement holder ring, there must be a groove to put it inside.



Thats the notch in the movement holder it holds in place:


Without this piece the movement shakes a bit inside the case after putting the watch together.

 

Thanks for the memory jog.
It must have been in that rinse jar for more than 12 months as that's the last time I stripped a 6139.
Was in the final rinse (denatured alcohol) that I only use after cleaning with water based cleaner (Simple Green solution). I found the water based cleaning to be better than L&R etc when doing cases/bracelets that had a lot of "environmental buildup".

 

Definitely a Seiko movement holder wavy spring

 

It is surely very clean now