I just joined on purpose for this one thread. I've been looking for the same thing - confirmation of the California Air Tools exhaust hose fitting thread. They didn't return my email either.
I thought it could be either a 3/8 NPSM with 18 TPI or 3/8 BSPP (aka G3/8) with 19 TPI. These are both ball/cone seat swivel type connections. Typically the male end has a tapered/angled face, but on these compressors they just seems to end flat.
The connection at the tank on mine had a tiny bit of gasket (probably PTFE) but the compressor motor side had nothing, just metal to metal seal (which is what these things are designed for anyway).
Adapters and fittings are available on McMaster Carr (USA) and Green Line Hose (Canada), which is where I was planning to order from.
For reference my compressor model is 20020 - 20 gallon upright.
A photo I just took is below. I count 18 or 19 TPI with the photo. Unfortunately have no thread gauges and my vernier caliper has gone missing.