Sure to be the frame number, but you're going to have to contact the Velocette Owners' Club to verify what I am about to say.
I have a number of Velocette books, including Ivan Rhodes' "Technical Excellence Exemplified". In the back it has a table of engine, frame and gearbox numbers with invoice dates.
Under a sub-heading that says "Larger percentage of KSS, KS, KT, KE plus odd KES" (presumably in the works records at the time) it has frame number 1436 for Dec. 1927, and 1461 for Jan. 1928, so let me guess your frame KT1440 is from a model KT of 1928. Manufacturers started making "next year's model" in August/Sept the previous year. That said, Rhodes lists various frame prefixes, but "KT" is not among them.
Anyway, the dating table is somewhat cryptic. There is a bunch of 1xxx frame numbers in 1931-2 (looks like a mass typo to me), and also the short-lived 1930 model KTP had its own frame number series, starting at 1, but it doesn't look like that got as high as 1400.
I reckon 1928, but better check with an expert!
Cheers
Leon