The District (Metropolitan District Railway) classified its electric stock from A (a couple of experimental trains) up the alphabet, and its sequence was followed on by London Transport; the large order for electrification was the B stock, which was followed by relatively small batches of classes C-E (pre WW1), then G and K-N post war, into the early LT era - all except M had manually worked sliding doors, and were, to a large extent, interchangeable, forming mixed sets (there were various modernatisation projects, including changing cars from motors to tailers (and v-v? - the story is too complex for me to remember details). F stock was the exception - 100 cars (so 12*8 car trains) to completely different design, and always kept separate. One of its distinct features was the oval window in the car ends. It was converted to power worked doors just before the war.
In the late 1930s, under LT, there was another reorganisation, and - apart from the F stock - all the District stock was divided into two groups: the Q stock, which had some new flared sided trailers (Q38), and old, straight sided motors (and some trailers?) converted from classes G-N (which became Q23-Q35), and was fitted with power doors; while the rest retained hand worked doors, and were classified H - these lasted into the 1950s. Post war, there was new R stock (again with flared sides), some all new, some rebuilt Q38 trailers (as motors), with some of the newer H stock being fitted with power doors, and reclassified Q. Also at that time (early 1950s) the F stock was moved to the Met. That is a very short summary of a long and complex story... (and the story of the earlier Met stock is even more complex...) See
https://en.wikipedia.org/wiki/District_Railway_electric_multiple_units (and links therefrom) for more...
