Although by 1915 Triumph were very busy producing Models H for the British and Allied Armies in the 1914 to 1918 war, the sale of motor cycles to civilians had not, yet, been prohibited. So Triumph continued to produce what it was able in order to satisfy requests, especially export orders, which would have brought much needed money into the country.
Thus 1915 saw the continued production of the Type D, TT Fixed-Engine Roadster; Type F Fixed-Engine Racer, plus a new, and now little known, Type K, TT Roadster with Sturmey-Archer countershaft gearbox, as fitted to the Type/Model H.
As is so often the case when 'a line is drawn', and in this case 'the line' is between Veteran and Vintage machines, the first two machines, above, are really 'Veterans', although their manufacture date makes them 'Vintage'.
At the end of the 1914-18 war, once Triumph had 'sorted themselves out', it was seen from requests from overseas that there was again a demand for 'sporting' models. (In Triumph's major export market, Australia, road and circuit racing was popular, while 'just across the pond' in New Zealand, Reliability Runs, Beach Racing and Hill Climbs were the popular events.)
Triumph thus re-introduced the Type/Model D, but due to the war, and strikes after (not at the Triumph Works!) the price had more than doubled. Although by then the standard engine was the 550cc 4hp type, as used with the Type H, most requests were for the fitting of the earlier 499cc 3½hp engine. Not only did machines then fit into the 'right' capacity catagory, but separate inlet and exhaust cams meant that the engines could be tuned, while there's not a lot one can do with a single-cam engine.
The Type D was not mentioned in the 1921 and 1922 home-market sales catalogues and appears to have died a natural death when the 4-valve Model R became available in 1922. My latest recorded Type D was assembled during December 1922.
1915 - Type D, TT Roadster Fixed-Engine, 550cc 4hp 85mm bore X 97mm stroke.
.............. Optional new 499cc 3½ hp 81mm bore X 97mm stroke engine.
........... Type F, TT Racer Fixed-Engine, new 499cc 3½ hp 81mm bore X 97mm stroke.
........... Type K, TT Roadster, 550cc 4hp 85mm bore X 97mm stroke. Sturmey-Archer 3-speed countershaft gearbox.
1920 to 1922 - Type D, as 1915, above.
I am unable to determine the production of these models as engine and frame numbers were concurrent with Models H.
Types/Models D still survive in Australia, Britain, and New Zealand.
For more details, technical information, performance figures, and much more,
you really need to refer to the Model H booklet I have written, which includes these models.