The early (like 1950's) Sun tachs used a sending unit powered by Mercury batteries that are no longer made, with no suitable substitutes. The sender looked like a big voltage regulator with a brown Bakelite cover.

Later models eliminated the sending unit and the unobtanium batteries. Some vintage instrument restoration places used to convert the senders to solid state internal circuitry so the box would appear original from the outside.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!