I can't find any mention, (or evidence) for such a groupset. There's nothing at all on the Classic Rendezvous site and Joels Blackbirds pages on Magistroni say nothing of it. He's pretty much the expert on it.
Of note though, is the fact when Magistroni disappeared in the early 60's, a few years later OFMEGA started (
OFficine
MEcchaniche
Giostr
A), surely no coincidence. Ofmega did go on to produce entire groups but not until the 1980's.
Apparently Magistroni catalogues are almost impossible to find, so I think coming across sufficient evidence for a collaborative effort with GB and Altenburger and three Italians might be a very difficult task. Good luck
Rebour drawing of Magistroni's revolutionary bottom bracket design with annular bearings.