[spuggy]

PET shows 911-541-101-04 "windshield, tinted, heated" available for the 1974 MY (as well as "901-541-101-24 "windshield" and 901-541-101-34 "windshield, tinted").

Part number changed in 1978, 911-541-011-21 is "windshield, tinted, heated", and there is also 930-541-011-02 "windshield, tinted upper part: dark green, heated".

1973 lists only 901-541-101-21 "windshield" and 901-541-101-31 "windshield, green"

1989 lists 6 different variants of the windshield, tinted/tinted upper part and clear - for cars "with trim frame MOD.89" and cars without.

So 1974-1988 seems to be the years it was a stock option. Last time I got a quote for heated, top tint, 911 windscreen from a national auto glass franchise, it was $1000 as opposed to $250 (from memory) and special-order.