I'm referring to the tach itself. The only U.S. versions that I've seen on Yamaha products are cable-driven, not electronic. They use a cable similar to a speedo cable. On U.S. models the XT600 drive them from the cam gear, XT350's from the timing gear down low in the cases.
The dash faces, lights and housing appear to be the same, so perhaps Yamaha went electronic on Euro versions, or maybe even on their later U.S. singles as well and I missed the change. Would be handy to know.
If they are the same physical size as the cable-driven versions you'd also need the plastic mounting bracket from the XT as the tachs are not the same size as the indicator light module on a TW. Speedos are all the same and drop right in.