Wanted Coker seat post

looking for a seat post for a Coker big one. I believe it to be 15 mm and if possible able to fit a modern unicycle saddle like a nimbus or KH.

I believe it’s a 22.2mm post for the original Cooker Big One

I’m measuring inside of neck tube with mm calipers at 15 mm. The frame is being painted by a friend right now. I will measure it again when I get it back later in the week.

If it’s steel, it’s 22.2mm
If it’s aluminium, it’s 25.4mm