Hi again Chris,
The 15 was made for a long period from the early 1950s to the early 1960s. It comes in two basic versions, with a bandless cap (which I have only seen with nickel plated trim) or a single band cap (like yours, which is I think always the gold trim version).
David Wells website also tells me it came in a longer and shorter version. Parts are usually easy enough to find, the main problem is wear on the plated bits of versions with the gold plated trim. The nickel plated trim seems much more durable. Colours tend to be black, plus the usual red, green and blue marbles.
I've sold (actually more like given away!) two of the super green and black marble bandless types, fully restored, on ebay recently, the nicest of which is pictured here

Overall, as you say, they are an excellent low cost every day writing pen, much better value than many a modern pen in the same price range. If you like the bandless style, also look out for the model 17 which is quite similar but comes in more dramatic colours, known as red flame, blue flame or green flame.
Andy