One number per volume is SIGGRAPH ... conference proceedings, <1981>-1992.
SIGGRAPH ... conference proceedings number continued in 1993 by: SIGGRAPH (Conference). Computer graphics proceedings, annual conference series.
Vol. 24, no. 2 (Mar. 1990) and special issue in 1992 consist of the proceedings of the 1990 and 1992 Symposium on Interactive 3D Graphics, later issued separately.