General A/C questions - missing some components

As I recall my 69 Dart, the factory AC had just a thermo-mechanical expansion valve on the firewall. It had a "gas thermometer" that stuck in a well in the tube after the expansion valve, a thin copper tube like you see on older gas water heaters w/ continuous pilot flame. That made pressure for the diaphragm of the expansion valve, making a simple temperature feedback loop - lower temp choked off the refrigerant flow, higher temp flowed more. I think the compressor was turned on and off by a temperature controller in the dash. Most classic after-market or dealer installed under-dash AC boxes use the same controls. Later A's added the low-pressure cut-off switch and such. Many Sanden's have a port on the rear head for a small pressure relief valve (PRV), which might be smart. I bought one for ~$8 on ebay for a Sanden. I think they pop ~300 psig.