These are the seat rails, and they are adjustable, but there’s no lock on one side.
This relies on the seat frame itself to absorb a lot of force, and is a prime cause of seat frame breakage. I suffered this in more than one bucket seat car.
But this is only of academic interest as I am going to toss out this unnecessary weight.
This car is so small that nobody over 5’-0” will be wanting to move the seat forward! (They will want to move the pedals back if under 5’-6” or so.)
I’m just going to mount the seats all the way back and solid to the car.
But I might mount them with slots to allow for a small amount of assembly adjustment.
Originally the car came with four of these seat risers. Unfortunately (and I don’t know how soon or if ever the original builder realized this) they sent him four right seat brackets and no left brackets.
When I got the car the driver seat was mounted all crooked in the chassis because of the mistake.
How I temporarily corrected it is like this.
One of the brackets I cut and welded to make it opposite hand.
One of the brackets I heated up with a torch, and hammered it backwards over an anvil.
All four of them will go in the junk as I am not mounting the seats directly to the floor. There will be some sub framing similar to what I’ve showed in the photograph above.