It's an electronic tach and I was told it was out of a early '70s Mercedes. No sender as it's for a gas engine. It has a red lead labeled + and a green one labeled TD1 or something like that.
It's labeled on the can as being for an 8 cylinder engine but I think I can fine tune it for my six with the pot, if only it didn't slam into the peg at the top end.

Michael

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Usta haves '69 250/8, '76 280C, 1971 250C 114.023, 1976 450SEL 116.033
Current have, 1983 300SD 126.120