Thanks, Tim. I’ll take a look. 52 1/4” appears to be found on the drawings. I found this backseat through eBay under an obscure heading located in Tulsa , OK. It’s in Lawton, Oklahoma now at my sisters' place. It seems these are difficult to find, so I took a risk and bid on it - not being fully informed it was correct. I was the only bidder and local pick up only. My sister drove close to 3 hours one way to pick it up. Bless her soul.
Next step is to convince her to bring it to NH. It has the sheet steel backing and subtracting the frame member width it may be an AD 3rd row seat and close to 52 1/4” (hard tellin not knowin) after-all.