Nick - Here's a bit of added info as I have been working with Bill on parts "starter" number/application info for his Tech Tip. This info comes from multiple resources, including original Delco-Remy catalogs, GM Heritage PDFs , GM Master Parts books, and period aftermarket suppliers catalogs.
These 4 starters were the primary part numbers for the 6 volt starter that uses the correct drive for your 139 tooth flywheel (all will work for your application):
GM# 1107055 - Chevy Trucks 1948-52 (can also be used to replace many earlier year 6 volt starters back to the late 1930's)
GM# 1107061 - Chevy Trucks 1946-48
GM# 1107103 - Chevy/GMC Trucks 1948-51
GM# 1107108 - Chevy/GMC Trucks 1951-54
If you are looking for what appears to be a rebuildable core, I found this
GM# 1107108 starter begging to find a new home. Even if you don't try and rebuild it yourself (based on Bill's info above I think you could), you could use it for a correct core to acquire a rebuild form one of the vendors. Just me, but I think I would jump on this one and see if you can't make it yours.
EDIT NOTE: Please note that the vendor identified at this link has been reported as a confirmed scammer and is NOT to be trusted for any parts offered through that site!
