I was lucky enough to find an original VDO 6-volt tachometer for the Type 34 Ghia. I've been trying to track one down for years. It's a very rare original accessory part—I've only seen three of them in person in 22 years of looking, and I only know of a handful of others that exist. VDO Type 3 tachs are pretty common in comparison. It needs restoration but seems to be relatively sound. It doesn't appear to have been opened for repair in the past. Cosmetically, the biggest restoration challenge will be the cracked "glass."
It has a white needle and silver knob and escutcheon. That and the fact that it's 6-volt means it was intended for a late '65 or '66 Type 34. All Type 34 VDO tachs were 6000 RPM to my knowledge. I had never noticed before when looking at others that the scale is progressive—it expands at the higher RPMs.
The bevel in the back of the housing was to allow clearance for the windshield wiper armature.
It has a May 1967 date of manufacture on the back. The terminals are marked (from left to right): +12 (blank terminal), +6, 1, and -, where +6 goes to the fuse block, 1 goes to the corresponding terminal on the coil, and - goes to ground.
A VW technical bulletin with installation instructions for the Type 3 VDO tach is available here. Conventional Type 34 wisdom says that when a tach is installed in the clock's location the clock is then supposed to replace the in-dash speaker, but these instructions offer a slightly different take on this idea:
The installation and wiring of the rev. counter on the 1500 Karmann-Ghia takes place in the same manner. On this vehicle it is also possible to fit the clock in place of the insert for the loud speaker opening if a radio is not included.
So, at least in VW's view, with a tach installed you could have a clock or a radio but not both. In practice of course the solution is simply to relocate the speaker elsewhere.
When I have this tach restored for my car I will substitute an earlier brass center knob and escutcheon to match the other gauges. Red needle too, of course. I'll keep the original silver parts so it will be possible to revert to the '65/'66 style in the future. Interestingly, the center knob is plated plastic, unlike the early knobs which are solid polished brass. (Were all silver colored gauge knobs plastic? A late-model Type 34 owner would know.) It appears that the knob and escutcheon aren't unique to the tach, but seem to be parts that are shared with the other gauges. That will help.
More information on Type 3 and Type 34 tachometers is available here and here and here.