It is a chrome plated flat piece of bar with a scroll end (for a finger to pull) with a couple of slots to engage on the seat sliders. It fits into tabs welded to the seat mounting box and there is a hole mid-point to attach a spring, which connects to a tab on the seat mounting box, so that it is spring loaded, pull outwards, move the seat re-engage on the slots and it retracts locking in position, simple and effective!
Hope these photos help, it was a bit awkward without moving the car out!
These are durable parts so you should be able to find these secondhand, often turn up on eBay, or dealers that have s/h parts, or Beaulieu, which is now only a few weeks away.
Passenger side
Driver's side
Passenger side by transmission tunnel (rotate anti-clockwise for correct orientation)
Passenger side nearest the door looking downwards
Another view of passenger side by transmission tunnel looking downwards