The 66 250S is a great car.. I sold mine that I had for years to a fellow in Hayward a couple years ago...
About the trans.. It was the first that Mercedes built.. It has no converter.. Its a lock up clutch... On the front carb there is a small switch with some thing like points in it.. when you are at an idle it opens the clutch so it will idle at a stop.. when you step on it and it gets to 800 to 900 rpm the clutch closes and all shifts after that are made with the clutch locked up.. An early version of the luck up clutch that americans invented in the 80s.. (Ya right).. With the small engine Mercedes did not want to loose power thru the converter like they had been doing with the Warner transissions that they had been using..
The old 250 single cam engine was a hold over from the early days of trying to make the engine last for ever some thing that both RR and Mercedes were doing from the 30's . They burned oil.. Lots of oil.. right from the factory.. the blow by in the engines blew lots of oil thru the vent in the valve cover over to the carbs and the engines were set up loose with almost no.. or no valve guide seals.. So the whole thing was kind of a controled leak to the carbs where it was sent into the pistons and lubed every thing including the valves ... At speed.. 100 plus mph it was common to burn a gallon per thousand miles in those old things.. and around town a quart every other tank of gas... Those old engines are still running with hundreds of thousands of miles on them and when you tear one down it showes little wear..
These smoking engines did not go over well here and were only brought in a couple years.. then they redesigned the engine and put a 250 in the 114 cars.. However it was not the same engine that was in the 250S cars of 66 and 67..
Great car.. But finding some one to work on it if you do not will be a problem as most do not under stand the twin carbs with switches the run the clutch on the automatic trans.. and every one wants to over haul it because it burns oil... Like it did when new..
If you do your own work it is a great car to own.. and one of the best looking cars Mercedes ever built..