Would someone measure the diameter of the main jet holder on the DT360a for me please?
I’m after the gasket, mine measures 12mm, but the replacement part I ordered are 10mm.
I was told by my supplier, I must have the wrong carby on it.
Also, how do you positively identify the carburettor?

Carb ID numbers are usually stamped on the choke tower .

There are model numbers stamped in every original Yamaha carb I've seen.  

If I remember correctly, you actually have what we consider an "RT3" here in the USA?  In any case, on a flange mount carb the TINY number is usually stamped on the upper left edge of the mounting flange.  

On a spigot mount (DT360A US model), the number might be on either the front edge of the choke plunger boss or on a little flat spot on the right side of the carb just behind the mounting spigot.

My guess is the problem is you are ordering for a "DT360" which in the UK did first appear in 1973 but in the rest of the world and in all the parts lists I've seen is called the "RT3".   (I think the 1974 DT360 went to the smaller 125/175 jet holder...  the 68-73 250/360 had a larger diameter jet holder.)

One other clue is the RT3 was the last 360 to have a 2-bolt flange mounted carb.  

1974 DT360 went to a spigot mount.  

If you have the flange mount carb, likely it's not what most of the world called a DT360.

pic is of engine side DT360A carb. That bolt is ~10.96mm diameter. Uses a 12mm wrench for removal. I will note that I recently ordered a gasket and the wrong size was sent to me...too small I believe from an ebay source(surprise).  gasket on this one 16x11x1mm

Mark, interesting you mentioned 73 as the DT360 in U.K..  I noted same on Yamaha Product Website. 
global.yamaha-motor.com/showroom/library...area=&country=&year=

John, the original poster, is in Australia.

It's probably a typo in the online parts list. My Euro parts list shows the same banjo bolt, but with washer/gasket part number 286-14198-00 (ie the same as the earlier 250/360 bikes).

pabdt, my DT360A banjo head size is 14mm!

Mark, the first of the big trailies we officially got in the UK was the DT250/400B in 1975. We got a handful of the smaller bikes from 1973 onwards. Europe got a handful of the bigger bikes, maybe from 1972? The Euro 1974 bikes had some different colours - DT250A was Blue or Gold, and DT360 was black (looks very nice, actually).

Thanks for the clarification Alan. I have seen brochures from the Netherlands importer written in English that identify the 1973 models as the DT250 and DT360.  They look exactly like the DT3 and RT3 we had here.  It is possible they were not imported into the UK and only brought into Europe? 

It was late last night when I responded and I didn't research like I normally do.  I remember replacing a main jet holder on a 74 DT360 and it matched up with the 125/175 main jet holder.  I had another larger diameter main jet holder which I think was from a 250/360 but it didn't fit the DT360.  But it could have been from something else entirely as it was loose in a box of parts.

I also have a vague recollection that someone here recently has one of those "DT360" models that is what we'd normally call an RT3.  I didn't research that either.