TR4A SU Center Pull Throttles-Population Size

[N197TR4@cs.com]

List:

Does anyone have an idea of the history and population size of the TR4A with 
center pull throttle rods and springs, used with SU/HS6?

The TR4A came in with the Z-S because of supply issues, I believe.

Then came back in at the end of the production run with HS6 and center 
pulls???

Thanks...Joe (A)