Most of the early ITs had 36mm forks - even the 250s and 400s.
The other give-away is the axle clamp.
Early ones have a single 8mm pinch bolt (YZ250/400D, and maybe E).
Most have a seperate plate that's held on by four 6mm studs (IT250/465, YZ125H/J, IT175J/K etc).
There's also a third type from the late 70s that have two 8mm pinch bolts (YZ250/400F, YZ250G, probably others).
Anyhow, none of that helps you get triple clamps. I'll go and check later this morning.