I have a set of drawings from Smithsonian that are the same crappy 1950's Air Force prints that everybody else has, and they show that the A model had the same seats as all the other models. I have found that it is not unusual to find those loops gone and the wire guards for the fabric attachments to be gone, too. For some reason, it was a common mod and you'll see all sorts of Rube Goldberg seat backs put in there and attached in some pretty creative ways.
The fact that it was registered as an L-4 and not as a J-3 has nothing to do with whether it was ever converted, it just means that unlike most of them, it was not reregistered as a J-3. The Army and the CAA made it possible for you to register your surplus L-4 as a J-3 if you wanted to, but you didn't have to. There are plenty of them registered both ways.
They aren't hard to make. I had to make them for my H model. Check to see if it has the original weld marks. Most of them were just roughly ground down so the interior fabric wouldn't appear lumpy. If you look closely, you should be able to see them. If not, then go ahead and copy the J because they were built the same. If you need to see the drawings, let me know and I'll send you what I have on the seat.
If he's going to install them, hope he has the fabric off. It can be done with the cover on but it's a real challenge. Lots of shielding and wet towels. I had to reweld my upper window latch bar. It actually came out well.