I might be wrong, but I think JD is asking what year the external oil cooling system was offered on production 911s......... yes?

If so, I believe the system started with the '69 production year with an optional front small radiator type cooler w/small stub-line fittings until the '71 MY. Then for '72 they had the same optional small radiator cooler however they changed to the bigger stub-line fittings (making this a one year only cooler). For the '73 MY they changed to the serpentine/trombone optional cooler with the bigger stub-line fittings. The hard lines and thermostat placement changed as well, I believe 3 times until the '74 MY where they settled on the hard line and thermostat location for about the next 25 years.

Hope this helps and answers your question.

Cheers