Well they sure LOOK like 911 pistons-- the deep skirt with the tang that sticks down, the oil control ring below the pin. Even the oiling holes below the second ring are in the same place as my 80mm pistons.
But here's the rub. If they REALLY are 87.5mm, why? ST pistons were either 86.7 or 89mm. The 2,7 pistons were 90mm as of 1974. I'm not saying I know definitively that they aren't Porsche aftermarket but it does seem unlikely that somebody would tool up to produce a smaller version than what you could get from Mahle.
Nor do they meet the description of any of the Big Bore 1720cc 912 or Type 4 stuff-- a 1720 is an 86mm bore.
So I got to wondering, where would you find a set of six pistons that was 87.5mm, made in the early sixties with the oil control ring below the pin with the long tang? Also, the "STD" stamp suggests that they are domestic production- that's English, not German. No markings of any kind? 87.5mm is the exact size of one of the early Corvair pistons.
That said, a search of the Internet (great thing) suggests that Mahle made a set of 87.5 for "spec racing". . . that is certainly identical to the Mahle design. But they are not "long tang" . . .
http://www.pbase.com/9146gt/my_9146_gt_project_my_engines_engine25ltr_mahle_87 5mm_pc
The mystery continues