This will be a little rough because I haven't looked at the seat adjuster in my 49 GMC since the cold set in. But seat adjustment on pre-'50s is completely different from what followed.
There is only one locking device. It's on the seat's driver side. Both seat rails clamp onto a cable that runs from the front of seat-base sides around the back to form a long loop. The loop goes through a couple of pulleys riveted to the seat base to route the cable from driver's side and back. There is an adjusting slide on the driver's side of the seat base so you can snug up the cable once it's threaded onto the pulleys. That done, you clamp onto both sides of the cable.
When you adjust with the driver's side device the seat rides back and forth parallel on the cable.
Problem. If it's a simple loop, the left side goes forward when the right side goes backward. My truck's PO didn't know this, so my seats don't move at all. What you have to do to get the cable running the same way on both sides is to cross it in back, so that the forward-moving inside of the loop goes where what the reverse-moving outside of the loop would be. That way both sides move in the same direction.
Try it with a loop of string and a couple of pencils to see.
The (good-for-nothing) seat cable on my truck is about 1/8" stranded stainless. I haven't changed it yet, but I'm told that Loew's and Home Depot carry it with connectors, etc. Or you could just visit the local hardware.
Good luck.

Never start vast projects with half vast ideas.