Hi Dali,

the strap numbers come from "The Catalog", the only known document which had almost all Gruen watch models from 1922 to 1958. The standard mans models are the Strap line and they are simply numbered from the very first in 1922 to the last in 1958. It was recently digitally remastered by Mike Barnett and is available through Lulu.com -> http://www.lulu.com/shop/mike-barnett/the-gruen-watch-catalo g-economical-soft-cover/paperback/product-22412671.html
Afire is correct with the solid green gold version being Strap 17 (Although the catalog says yellow gold), the white one is the mentioned Strap 24. There is also a version in Sterling silver which is Strap 15.

Barney