Based on the 2002 Gitzo catalogue, the #37X series heads are rated at 13 lbs capacity. From personal experience with an older version of this head, Rationelle 3B from the early '80s, it holds my 4x5 Technika, heaviest lens, and Grafmatic holder (just over 9 lbs total) with NO PROBLEMS at all on either my 3 and 4 series Gitzo aluminum tripod.
I think with the black plastic bts the head is newer than you're suggesting. Mine has grey handles and knobs and bought it around 1982 or so.
You can also use a 3/8" screw on it by swapping out the 1/4" on - they were standard Gitzo accessories.