Well.. if the tach is for a 122, then it *should* have a long wedge foot, but that's not absolute. My 6-cyl tach pod from a 164 had just a domed bezel. a plastic spacer ring, and a big plastic wingnut to secure it. I put that in my 220, but have not yet hooked it up or adjusted it.

My 68 sedan (22 years in storage) has a working 4-cyl tach pod, and it is identical to the 164 pod, except that it is calibrated for a 4-cyl engine. So no foot, just a domed bezel and supposedly stock.

I've not puzzled out the wiring, but there is one wire for a bulb inside, one wire to the coil, and two (white?) wires that attach to either side of the fusebox.

The back of the pod comes off (to access the internal securing screws), and the front half of the pod comes off to access the innards.

Let me know if you need further info on wiring and I can investigate my own some more.