1946 Waltham wrist watch

[thecodedawg]

I have a 6/0 waltam wrist watch that I just found at a thrift store.  I have disassambled and serviced it, however, it has a broken mainspring.

Width is 1.521mm, thickness is .115 mm and the length is 308mm or about 12"

Cas-Ker has what looks like 2 options differing in only dennison strength.  After some reading, it appers the dennsion strength is a measuremnt of the thickness of a mainspring. That thickness is proportional to the inner diameter of the going barrel.  Either I will need to get a measurement of my going barrel to figure out the dennison strength, or I just order the 10-3/8 over the 11 since this is a 17j movement and probaly doesn't need the strongest mainspring.

What mainspring should I choose and why?

A couple of pics.  So far, I am enjoying working on Waltam movements. This one was a breeze getting the wheel bridge and the barrel bridge lined up and in place. 

[WatchMaker]

And the image below is more evidence that Waltham specified two different strength springs for your model.

Your movement came in 17 and 21 jewel varieties and my take is that to compensate for any slightly greater frictional load in the 17 jewel variety, Waltham specified a slightly greater strength over that for the 21 jewel. The stronger (thicker) variety is the one I would opt for.