I have this 21-jewel Longines private label watch that is currently being worked on by Roger N. We need to find out the specs for the mainspring before he can finish up the project. When I first bought the watch I thought it was 18s, but when I found the illustration of the movement in the 1951 Paulson's Master Catalog, it appears to be a 21 ligne. The serial number of my watch is 734,564.

Here is a photo of the actual watch. It was a contract watch for F.M. Porter in Albany, NY.

Here is a pic of a deck watch using the same style movement. I would assume this is chronometer grade and from a later date.

Here is another pic found on the internet (much later serial number than mine). If someone has an old Longines parts catalog indicating the L, W, and S of the mainspring I would greatly appreciate hearing from you.