Do the early DT1:s use an o-ring on the carb inlet to cylinder side? There is a groove but the parts listing do not show one.

As far as I know, yes! All other carbs of that year have the same channel and show an o ring. That’s odd it’s not listed on partzilla. Granted, I don’t have a DT1.

Early didn't have one according to posts from experts here.

First year they start showing one is 1972. Odd that they have the groove for it but don't use one.

Yes the early DT1-250's require an oring in the carb.   VMS Speed and Sport Parts Store (speedandsportinc.com)  list the oring item number 7, 214-14147-00-00O-RING



The other vendors may not have the orginal parts manuals.  

That is the o-ring for the main jet circuit.

Yes, all the flange mount carbs have a very important o-ring (#7) inside the carb to seal the float bowl to the jet setter... 

To clarify my answer, I read the question as asking about the o-ring groove in the 2 bolt carb mounting flange.  O-ring would fit in the groove in the mounting flange right above where they've drawn #2 and #3 in the parts diagram posted, if the early bike had one.

I've searched the parts diagrams...  carb and "air cleaner".  (Intake manifold parts are often shown in the "air cleaner" or on later bikes "intake" diagrams).  I've never found a diagram that shows the mounting flange o-ring for an early 250.  Was it an oversight or mistake in early Yamaha parts books?  I'm not sure...  but I have seen experts on those models post here saying the early models only had a mounting gasket and no o-ring.

Sorry about the confusion I did not read your question closely enough.  I have an Oring in mine on the inlet between the cylinder and carb.  I have used just a gasket with Indian Head gasket sealer, both work.  In my opinion the Oring is more efficient especially when you need to pull the carb and then reinstall.  And yes you are correct that early parts manuals did not list the Oring.
 

Mark is correct , pre 72 the O ring wasn't installed ex factory .
I have fitted one to my 69 DT1 , as I figured anything that might assist proper sealing of the intake flange ,
wouldn't be a bad thing , even if not historically correct .

Yes, My '69 DT1B has one where the carb bolts to the head.  Take the carb to any auto parts store and pick one from their o-ring kit.