You can repair these fairly easy yourself. Found an article several years ago on Plymouth site:
https://www.ply33.com/Repair/tempgauge
Basically, go buy a cheap mechanical gauge (Harbor Frt., PEP boys,....last I bought was NAPA to get the wire wrapped look on capillary tube....think it was $22...).... system is filled with ether...you condense all into the bulb with ice...with bulb in ice, cut the capillary line close to the gauge on both, using a brass tube/sleeve (from hobby shop) solder them together (have to keep id open for fluid). pull bulb out of ice... can test by putting boiling water and checking reading...
The key is you're working on saturation curve for the ether... at a given temp it has a pressure, will be constant until all the liquid is boiled off. If you don't boil it all off, press/temp will track the sat. curve, be repeatable.. any other "gases" are minimal, mostly non condensibles and insignificant to overall pressure, act as "fillers/spacers" in vapor space...just have to have enough "pure stuff" in bulb not to all boil off...
Works pretty trick. I'ved saved about 5 or 6 Smith's gauges since finding that article. Much cheaper than buying new gauge, and keeps the original face.
Bob L.