Yeah it’s a CSYNC cable from retrogamingcables.co.uk
As for the mod, I don’t know offhand. It looks like it can be set to either by setting a jumper, so I’ll have to open it up later to see.
Presently, it’ll sync up 99% of the time if I turn it on, and then reset the SNES after a few seconds. Seems like it settles on a stable frequency somehow. Not sure why resetting it works, but that seems to fix it, just powering it on is hit or miss.