Bruce,

There was no GM Chevrolet truck tach as an option or an accessory from 1947-55st

I have no documentation regarding a tach in a GMC between 47-55, but it is my understanding that a tach was available as an option on 1954/55 (Z-series) GMC 350 trucks. Here is an eBay ad for a Z-series 350 with a tach.
"Tach is a cable drive and works off the distributor, this is a very rare option."

The remodeled dash in 1954 (Z-series) GMC trucks had a hole/cover on the right side of the instrument panels - it normally has "GMC" on it, I think. The tachometer option would have gone in that hole. It looks like this truck's tack option is cable driven; but, there is no photo of the cable and the picture of the distributor is awful.

Tach photo - front

Tach photo - rear

Tach photo - distributor cable-fitting?

Please note that I have no GMC documentation on the tachometer being an option on the GMC trucks.

Tim
1954Advance-Design.com
1954 3106 Carryall Suburban
1954 3104 5-window pickup w/Hydra-Matic
The 216 and early 235 engines are not "splash oilers" -- this is a splash oiler.