The gauge is from egauges.com. Their number 150-101. See this link:

http://www.egauges.com/vdo_mult.asp?Type=Turbo_Boost&Series=Vision&Units=E

The 15 psi "Boost" gauge is $32.50, not including tubing, which is available in several kits or you can roll your own. The input to the gauge is 1.8 male pipe thread which easily connects to either 1/8 or 1/4 inch tubing.

This particular gauge family, the "Vision" series, has really good lighting, something that us old farts with our weak eyes especially appreciate. The Vision series is also a good appearance match for the factory gauges in the W123 cars.

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970