...I don't think I have ever even seen one with vertical mounts.
Well you can see a picture of one here:
https://www.douglasmotorcycles.net/index.php/topic,1057.msg3536.html#msg3536There are plenty of period photos of the Mark series Douglases, where the vertical flanged nature of the carburetor is apparent. As well as shown in the Mark series illustrated spares list:
Large imageIn there it gives the carburetor numbers as: 274AJ/4A, right-hand; and 274AK/4A, left-hand. These carburetors are not going to be available from Hitchcock’s as they sell new production of the older model Amal carburetors. The vertical flange style was only used by Douglas, so they are not common enough to be economically viable to reproduce. Well I say only Douglas used them, but I have heard there was a postwar vertical single that used the vertical flange body, so that they could lay the carburetor body over on it side. But it only used the one hand, being a mere single (can not remember the make and model) rather than “The Best Twin”. Other than the body, the rest of the parts should be interchangeable with the more common horizontal flanged 274. Note that the spares book gives one model carburetor as fitting Mark1 through Mark 5. This spares list was printed long after the Mark 1 ceased production, in the Mark 5 era, when originality was not a concern. They give parts that are interchangeable; at the bottom of the list they mention the original types: 74J/4A and 74K/4A. Some of the components may not be interchangeable between the 274 and the 74 series, you will need a carburetor expert to help you if you need to mix and match. One visible difference is that the T35, or Mark 1, had a different air intake funnel. It was capped on the end, the air entered via a series of holes drilled around the periphery. Also elsewhere on this forum
(link to post) (link to pictures) it is mention brass bodied versions are known rather than the more commonly seen aluminum (these brass carbs may be the 74 series, as they have the prewar style air vent.)
Of course, this all pertains to originality. It will run perfectly well with any suitable carburetor using an adaptor.
According to the Plus handbook, Plus models were fitted with the Amal 276EP/1B, right-hand; and 276EQ/1B, left-hand. Amal TT carburetors could be supplied as an option, as could probably anything reasonable the customer would ask and pay for! I have not seen a vertical flanged 276 Amal, or TT Amal. All period photos that I have seen of Plus models where you can see how the carburetor is mounted show a clip-on carburetor, with a flange to stub adaptor. That is not to say a vertical to horizontal flange adaptor was not available then or shortly after. I’ve just not seen an old photo of one yet.
There were some Mark series machines exported to the United States. According to the Works dispatch ledger my Mark 3 Sports was shipped directly to its new owner in New York City, in May of 1948. However there were not a lot of machines shipped to the States, forty-some I think between 1946 and 1954 (and at least one 90 Plus that a friend of the family bought after the New York City Motorcycle show.) There was an article by Jerry Rodgers about export numbers in an old issue of the New ConRod. There have been far more (Pre- and Postwar) imported by collectors and enthusiasts since, than by the Douglas factory.
-Doug
P.S. Alwyn beat me to the post while I was typing this up!