First, have you located the arm under the one seat rail that the loop handle should attach to? Your thread on 914world showed a picture of how it is when the arm is basically falling out of the rail. If you find it, push up on the arm while pushing the seat forward.
Here's a tech article about replacing the stock seat with an aftermarket racing seat; it has some pics of the stock hardware:
https://www.pelicanparts.com/techarticles/914_seat_install/914_seat_install3.htm
This is a pic of the seat rails from underneath. They are attached to some cross-bars that won't be present on your car, but these are what they look like otherwise:
The arm that the loop handle attaches to is the one swung up and out of frame in the upper-left. Note that the pic is taken from what would be below the seat if the parts were installed on the car.
Here is a photo of the loop in place. This is taken from the front of the seat, as if the camera were on the floor of the car pointing aft.
You would push up on the arm that the loop attaches to in order to slide the seat forward.
Once you get it all the way forward, you can push up on the tab under the other rail and continue pushing the seat forward.
Good luck!
--DD