When I was rebuilding seats/ installing headrests in my Amazon, I removed vinyl/ foam from both donor and receiving seatbacks. While the internals were slightly different with the 2 sets of seats (one was from a '68 model, the other from '66 or '67) all 4 seatbacks had 2 little round metal holes attatched to horizontal bar at top of seat. The headrest mounting hardware fit into those holes. The donor seat with headrests obviously had the mounting tubes, but the seats without headrests had no mounting hardware inside.

Once the long headrests (and little various plastic pieces underneath were removed, I was able to use a pliers (or maybe it was a vise grip?), grab the top of the mounting hardware (long metal tube) and pull it straight up/ and out from the seat back. The metal tubes which headrest goes into were *not* welded to horizontal bar in any way. Tight fit, but no welding, at least on my set of seats.

However, I also once tried to get the mounting hardware tubes from a 140, and they were indeed welded into the seat frame, making it impossible to pull out using above technique. Had to get a bar and pry/ break the weld to remove them. No idea if any of the seat innards had been messed with in the past, or if this is the way all 122 or 140 seats were put together.....