I sold someone an om617 instrument cluster and they want to return it because the back of the speedometer/odometer gauge is different and the ports don't match.

I've never seen a cluster gauge with ports like theirs, from a 1980 300TD. All the stock ones I've seen are the same.

Is this an aftermarket gauge?

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That is an electronic speedo. There are a couple odd years and models where the speedo was electronic even back in the 80s.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

That looks exactly like the cluster on my '81 300TD. That was the only year they used the electronic speedometer on the U.S. spec wagon with the 722.303 transmission. Same as was used on the SDs of that period.

You can actually read the date stamped on the back of the speedo.

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1981 300TD 310k miles
1970 280sel 172k miles
1966 230 Fintail 162k miles

"Where are we going? And why am I in this hand basket?"