Laugh and Learn.
One critical dimension for Plunger Seats is that the two seat spring mounts match the frame mountings. Back then with limited choice I measured the frame, found a seat frame to match, powder coated, re sprung, new cover etc. Came to fit it......too wide. The mistake I had made was to measure the bare frame. With the rear wheel in place the frame was just that bit narrower. So I have one spring vertical, one leans a bit. With that bit of wear on the seat front frame pivot, sometimes it feels as if the seat is alive with a mind of its own. You'll also damage that nicely finished rear mudguard when the seat frame rear rail smacks down onto it at the first pot hole......
Swarfy.