Attached is a picture of a part attached to the A/C lines on the back of the compressor. I suspect it is some sort of pressure relief valve. The photo is looking at the compressor end of the lines with the discharge port on the right nearest my mystery part. If someone can confirm it is a PRV, it would be appreciated.
Incidentally the new lines did not come with this part and it had to be transferred. I discovered this after discovering the hole was filled with a not very tight nut. Very messy after pressure applied!

You are correct. Are you sure that the auto parts store gave you the correct lines as a replacement?

Yes, they have R12 fittings and the behind the engine routing that was discontinued in 93. I just installed them without comparing the removed lines with the new one. I didn't critically look at them until after I tried to pressure up and the loosely inserted plug and o-ring blew.

Did you lubricate the O rings before installing them?

Yes, I used new green O-rings and lubricated them with the PAG oil. There are there are multiple O-rings on the spring lock couplers and one each on the accumulator, evaporator and orifice fittings.

I learned my lesson when installing the injectors when I replaced the heads last month. Not knowing what to use I just used gasoline. After priming two of them blew flooding the top of the car with raw gasoline. I reinstalled using motor oil and had no further problems.
This working on car stuff can be dangerous.