Ah, so you got that one! I hope you find it a good home.
That one has the Chronometer Balance and full jewelling. Top quality work from Gruen. Supposedly there is a 15 jewel variant, but I've never seen it. The chronometer balance however comes and goes in this caliber family (known caliber numbers 21, 35, 45, 50, 52 & 56 - anyone know any more?).