The newer seat design is significantly larger, on account of the bigger fuel tank introduced in 1972.

The previous version was smaller and only fitted on the K1 through K3. Mine came with a replacement cover and, unsurprisingly the 55 year old foam was showing its age:

I cut out the crumbly bits and used upholstery foam to repair the gaps . In what I suspect is not a conventional upholstery technique I used coarse sandpaper to get the new foam back to the original profile. If I was doing it again I would have used denser foam which would - I think - be easier to shape.

I was very pleased with the replacement seat cover - from replica-store (Thailand) on ebay. The cover was £45 delivered and arrived inside a week. It is well made, with a stitched seem used to hook the cover on to the "teeth" on the inside of the seat, and foam padded pleats like the original. It fits perfectly.

The aluminium trim is not included with the cover, so I got a nylon version from P & K Classics in the UK. Easy to fit: there are 4 holes in the seat base and you slot the the heads of the included bolts into the trim, poke them through the holes and attach the washer & nut on the inside of the seat. If you've got a half round woodworkers gouge you can create a neat curved cut on the ends, as demonstrated here.