Mid-80s. I had an '85 740 turbo with the intercooler, I think they brought them out in '84 or so and built them for several years. I don't think they put the intercooler on all turbo models though, look for the "Intercooler" on the trunk lid. I'm not too up on Volvo stuff anymore so I'm not certain about the years.

EDIT: An Ebay search for Intercoolers turns up some interesting possibilties...