I confirmed the following in use at Mosport in Ontario in July.
451.17500 165.5 PL IMSA Race Control
451.48750 203.5 PL IMSA Safety Vehicles
I was much more interested in the safety crews than race control because the action appeared to be happening there.
The participants included:
Control - the net controller in the race tower.
96 - the doctor/responder in the medical car. Also appeared to have control over the other crews.
Course Prep - Mosport's crew of tire wall repair/track service workers.
Rescue # - IMSA fire response crews identified by turn number (i.e. Rescue 2 was at turn 2).
Response # - Mosport emergency responders identified by turn number. These were in non-specially-prepped vehicles (i.e. SUVs donated by local dealers). Response and Rescue were rarely doubled up - that is, Response units were only present in turns where there was no Rescue.
Wrecker # - Local contract tow truck working for Mosport, parked at turn #.
Rollback/Flatbed # - Local contract flatbed tow truck working for Mosport, parked at turn #.
Ambulance # - Mosport ambulances (track-owned and operated), parked at turn #. Total of four. "Road Ambulance" refers to the contracted municipal services (2 total) that were parked outside the track to receive any patients that needed transport.
Pit Boss - the IMSA (I suspect) official in charge of the pit lane.
Medical Center - hopefully obvious, the "field hospital" at the track.
There were a few other call signs but they were relevant to Mosport only.
The corner workers and flag stations where yellow flags/etc were displayed appear to be on a closed-circuit, wired loop instead of a radio frequency.