We know that Siffert was getting Heuers from Heuer at or around wholesale price and turning them around to fellow drivers (and anyone else around the F1 circus) at probably a bit below retail but still enough for him to turn a profit. It wouldn't be too much of a stretch to imagine him driving a 911 for a year and then moving it on, if he could get them cheap enough in the first place. Jack Heuer describes him as a "natural wheeler-dealer", and the name certainly seems to fit.
Even though branding was just taking off in F1, I still find it an innocent time compared to the more cynical product placement of today, which sees drivers strap on the brand of the team's choice after a race. Somehow though I can still see Smokin' Jo making a go of it, trying to flog his Heuers to the various team bosses!
If you do a search for 911 here, you'll see there are a fair few dotted around the readership. And at least 1 912...