I think it's because people are unsure what you mean by 'seat pad', your pic shows the carpet and the seat 'rest/retainer loop' but it looks like you're on about the carpet.
If you're on about the retainer loop things, they are bolted down through the floor and need to be undone from underneath the car.
If you're on about the seat tilt bracket ( which i don't think you are as they are not in the shot) they bolt in from inside the car as you can probably see.
Thinking about it, are you on about the small metal plates that the curved seat back 'feet' rest on? It seems likely, these are about 2in x 4in plates, about 2mm thick(painted with that thick black paint that flakes off in big 'plastic-y' chunks when its old), with one edge curved up slightly to match the floor pan, screwed in with 4 self tappers.
I should have a full set or two kicking around ( my HB SL is in boxes, literally) if i can dig them out i'll send you pictures and measurements etc....I do know someone who might have a set going spare as not many of his cars have standard seats so i'll make inquiries there for you.
As for the seat cable thingy, you could do it 'in-situ', but it's so easy to take the seat out i'd do that, save your back and do the cable on the bench