Not sure what Peter means by OEM part because I doubt that SBIG would manufacture or request a part outside the standard 2.1mm or 2.5mm. Whilst the Switchcraft describes the S760K as a 2.03mm the datasheet indicates 2.1mm.
http://www.switchcraft.com/Drawings/S760K-S765K_CD.PDF
The Adapter that was supplied by SBIG with my ST-8300M is a Cincon Electronics TR45A12-01A03. If you do a search for this part number it throws up that the plug is a 5.5mm/2.1mm.
Soo...go with the 2.1mm....or buy both sizes and trial fit with the jack on your camera.