I was nearly done assembling this post (a post I started when I spotted the useful seat comparison image below) when I discovered that Derek already has a pretty complete post (and frankly better than mine was going to be) on the topic of rear seats (and was the actual source of the seat comparison).
But, there were still a few unanswered questions I had after reading his page, so I deleted what I had and reframed it as shown below.
I realized:
1) I didn’t know if the DJ-3A used the CJ-5 rear seat, the CJ-3A/CJ-3B rear seat or something else?
2) And, did the Surrey have it’s own rear seat or the same as the DJ-3A? (My guess, based on the pics above, and Bruce’s pics below, is that the DJ-3As and Surreys used the 3A/3B seat)
Below are photos of Bruce Agan’s DJ-3A restored back seat:

While my DJ-3A came with a rear seat, it was actually one of the folding rear seats from an M-38 or M-38A1, a topic covered in these two links:
https://forums.g503.com/viewtopic.php?t=235427
http://willysmjeeps.com/v2/modules.php?name=Forums&file=viewtopic&t=10636
Here’s a pic of my 1956 DJ-3A with the rear seat that was in it. While no the correct seat, it fit in there pretty well:
