In 1994, new constrction, a Honeywell Trol A Temp Zone Control System was installed with7 day programble Honeywell model T8600 Chronotherm III thermostats. The master thermostat on the lower level was recently replaced with a Honeywell Chronotherm IV model T8624D 2012 and now I need to replace the original upstairs stat which is a slave to master.

Is there a compatible thermostat available. If so what is the model number?

What is the brand and model number of the thermostat upstairs?

Upstairs stat is Honeywell model T8602 Chronotherm III

For all questions concerning this thermostat, please read and follow the instructions. If additional
Assistance is needed, call Honeywell Customer Assistance toll-free at 1-800-468-1502, Monday
Here is the reason you need to contact Honeywell.

T8621D7063 Trol-A-Temp® by Honeywell logo; system switch:
HEAT-OFF-COOL; O and B terminals.
None See Trol-A-Temp® representative
for replacement.
T8621D7071 Trol-A-Temp® by Honeywell logo; no system or
Fan switching.
None See Trol-A-Temp® representative
for replacement

It says there are no replacements available but I believe there is. Call Honeywell.