Brian is correct, the Series code '3A' was not stamped on the frame rail (at least not during the TF years).
Measure the wheel base, if its 114" it's a 1/2 ton 3100 short bed. If it's 123" it's a 1/2 ton 3200 Long bed or 3/4 ton 3600 (they never made a 3/4 short bed)...the 3200 and 3600 share the same frame. Looking at your pictures in the first post the rear spring hanger looks pretty close to the cab making it a short bed. A picture of the side of the truck would confirm this.
Mike B
