Posted this on the cabe but not really their thing. I can not figure out how to remove the old seat frame on this seat post. I would say it's not possible but what was left of the seat was a Viscount 2123 which was not the original seat so someone did it.
I had one of those posts once. When the bolt is fully loosened, that bottom piece that the rails sit on should slide out. I may have had to persuade it to open up a bit more with a screwdriver.