Which post is connected to the battery should matter in this case; see below. (Diagram is for '48-'49). One side of the pull-in coil is fed by the battery, the starter button grounds the other. Actually this answers the question why mine works as-is; my starter button is set up to send power to the solenoid (it doesn't ground the connection). Hmmmm.... I need to get under there...

 

Update: I checked my solenoid with an ohmmeter. It must be a late-50's model, i.e., for 12v. The two big terminals are interchangeable and completely isolated from everything, neither feeds the pull-in coil. I have to send +12v to the solenoid terminal to get anything, and it is grounding thru the mounting tab. This solenoid was on the truck when I bought it, looks pretty old, and was in use when the truck was still 6v.

Moral of the story: PO's do many wonderful things to vehicles. Before you assume what you've got, check it out!!

 

I found the same thing when I checked my solenoid, and the evidence showed when it started as a negative ground. So I'm guessing mine is also newer than 1948.
I bench tested my starter and it's not working. I don't know much about electric motors so I'm going to find a rebuilder tomorrow and hopefully get some positive results. I can't wait to get my truck running again.

 

The original 6 volt starter solenoids (21A-11450) were quite helpful, as they have a starter button on the bottom. The solenoid changed in 1952 (FAB-11450-B originally~B5A-11450-A now) and again in 1956 (B6A-11450-A originally~B6AZ-11450-A now [Motorcraft SW-3]). All the solenoids originally were marked with the Ford script. In the 1960's the mark was changed to FoMoCo also in the old style script. Those days are now gone, as the script has been removed, and the solenoids are naked as jaybirds today.