Settle it by asking the owner what the rpm is at 60mph (I don't know what it should be, sorry). I'm not sure if shifter bushings could give you enough play to indicate "D" but actually be in "3". If that's the case, would lever in "N" put you in "D" (4th)????

Alternatively, for $500 plus a rebuilt trans you'd have a nice driver.

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Hanno
'79 6.9 Sold (after 27 years)
'83 280SL, 5 spd.
'94 E320 Sdn. 5 spd conversion
'02 E320 Sdn.(on loan to mom!)
'87 300E (5 spd. conversion) Sold
'05 E500 Wagon