As far I can tell it is a Watson 30 series, which preceded the 40 series. Which was used on lighter trucks like 1ton and 1-1/2ton. I found a Watson Model 31 it was a 2 speed which could be mounted in either orientation for Direct and Over, or Direct and Under.
Also found a Watson 39-M1 which is a 3 speed like mine, but does not mount directly to the trans, but rather just behind like most other aux boxes.
Hopefully the ATHS (AMERICAN TRUCK HISTORICAL SOCIETY) [http://www.aths.org/] will be of more help...apparently they have a Master parts manual for Watson Aux Gear Boxes.
Watson Aux Gear Boxe where used by all the light and medium duty truck manufacturers, i.e. Chevy, GMC, Ford, IH, Dodge, Reo...etc. etc. mostly pre-1960, but not all.
The one I have is perfect for splitting gears on either the SM420, SM465 and the Clark 205V & 207V, it works in true progressive fashion mounted behind the main box. I was just lucky enough to find one that is married directly to the main box, my SM420.
I will add another picture later tonight.
All Hail Klaatu & Gort
