Jeb--suggest try R. K. Leightons in Birmingham UK.
The phone will probably be answered by the son of the founder, Andrew Leighton.
They supplied seats to most of the Brit bike factories "back in the day".
What Andrew doesn't know about seats is not worth knowing.
And they are happy to supply direct to US--I have bought several seats from them.
HTH and A Happy and Healthy New Year to you!