Originally Posted by vsbabolat
I think it was head size. Because there was a GC-30 that was larger.
Right you are! - I had a buddy with a GC-30 (with blue graphics), roundabout 1983. That model looked to me then as the Spalding analogue to the Head Graphite Director, with the GC-20 being their direct mark to the Graphite Edge (although in my humble opinion, the Head models had a plusher feel and were more stable, albeit slightly less manoeuverable - by dint of their narrow and less substantial shoulders, the Spalding models felt somewhat "floppy" or more sensitive to torsional twist on off-center hits).
Seems like lots of racquet model names in the 80's had numbers equivalent to the percentage the hitting area was larger than "standard": Rossignol with their 100/150/200/300 (drop the final zero and you have the rough % larger than standard), the 1986/7 Kneissl lineup with Masters 10/25/30 and Spark 25/35, etc. ...