861 fourth wheel question

One for the sages out there. Just serviced an 861 from a MK2 Speedy. Changed the fourth wheel as it was bent. Used one from my spares stash. Didn't think to count the teeth. Timegrapher is showing perfect timing, hands say otherwise, gaining several seconds a minute. Can only assume the tooth number is different, so, what calibre could the wheel be from? I did note the sweep second was a little snug so presumably larger staff, which I put down to parts variance.

Having had a bit of a ponder following the, "oh arse" realisation. I'm assuming the 321 must have a very similar 4th wheel, but for 18,000bph not 21,600.