My '74 850 Commando came to me as a HiRider...which I quickly preformed an emergency Roadsterization upon. Had all original turn lamps, horn, centerstand. I too thought the horn to be non-functional, so removed it but then discovered it worked very well once connected to good power source (a poor battery and/or poor ign switch power source likely played a role of non-function in bike).
Forks are standard length across all Commandos.
My bike has dealer installed 16 inch fatty rear wheel...an attempt to further increase the "Easy Rider Chopper" look. I replaced with 18 inch alloy that I spoked to the hub myself.
Other known differences from other model Commandos': the front brake hose is longer, as are the switch cluster wires I believe and clutch cable, to accommodate the higher handle bars. The headlamp is the 5-3/4" size, not the 7" found on the other models. The tank is the 9 Litre "peanut" size (mine is metal). Obviously the banana seat and tall grab bar is H.R. specific.
That's it.