Firestone made both ball lock and pin lock
Cornelius made both ball lock and pin lock
John Wood made both ball lock and pin lock
The physical dimensions alone won't tell you the manufacture. I'm sure that all three companies made the tanks for the buyer (coke or pepsi) based on the buyers specs. For example, kegs for coke had to work on their automated cleaning/filling equipment so I would expect that the height/width/location of posts etc. had to conform to that equipment.
Cornelius made ball lock
- Spartan
- Super Champion
Cornelius made pin lock
- R
Firestone made ball lock
- Challenger
- Challenger V
- Challenger VI
- Super Challenger
Firestone made pin lock
- A
- R
For the sake of completeness
John Wood made ball lock
- 85
John Wood made pin lock
- RA
- RC
Edit: From the sounds of it I think you do in fact have a Cornelius R that has had the pin lock posts replaced with ball lock posts.
Edit #2: Both the ball lock and pin lock kegs from Cornelius had the same size/pitch thread (19/32" x 18) so the posts would be easily interchangeable.