From the eBay ad:
FITS: SUZUKI GS750E/L "L" 1980-82, GS750S KATANA 1983, GS1000E/G/L/S 1978-80, GS1000S/1100S KATANA 1982-83, GS1100E/ES/L "L" 1980-83, GS1150E/ES 1984-86
All those are TSCC motors. The B12 uses SACS motors. The SACS motors were developed from the TSCC family, but few, if any, parts interchange... however, the port spacing on the heads may be the same, but if you wanted to go with a drag header, you might want to consult someone who drags their B12 with this style pipe and see if it can me made to work or not.
Maybe
Brock Davidson knows?