I recently had a chance to compare my BB heater box to a SB. I thought they were both the same and the only reason the BB box had a different part number was because of the hose cover or clips or heater core or the rubber hose hole cover. So, I tried putting the core from the BB in the SB box. It did not fit. After closer examination, I found the one thing that is different between the 2 type boxes. The BB welded in heater core plate is approximately 1/4 in. wider. I could probably make the SB work with a BB core, but I would have to modify, I.E. hammer, to make it fit. Below is what I found. I hope this helps someone ID their box in the future. Bottom box is the BB one.
