Thanks, yes that post is the one I was looking for. When I got home I was able to find my file on this Dutch gun and compare the marks with the similar one in storage in the V&A museum thanks to their website's details. I was a little worried that somehow one had been stolen out of the museum, gone to auction, and eventually bought by myself, however unlikely it would have been to have a major public auction of an item in the same town where it was stolen.
The answer came in the tiny detail of what are apparently the castle's inventory numbers stamped into the underside of either cannon. The number on the (assumed) underside of the one in the V&A museum storage cannon, no. 39-1887, is "IV" as in Roman Numeral four. The same number on mine is "I." That and the engraved weight in Dutch pounds is the only difference between the one in the V&A and mine (well, they have a ramrod and I don't.) Even the fancy tompions are identical-a cast bronze disc at the front, backed by a turned wood plug, held in by a piece of spring steel. I'll look for my photos of that gun today and post them when I can get them scanned.
So it looks like there are four of these similar cannons. The two still in the Netherlands are the same except for a difference in the coats of arms, one for each of the two families that was joined by marriage (?) I'm guessing those are marked "II" and "III" underneath-don't know if I have that in writiing anywhere however.
The remaining mysteries about this cannon for me to follow up on someday when I get lots of time to do such things include:
-What do the cast-in initials "JS" stand for on the bronze tompion?
-What can I find out about the carriage maker, whose cast bronze tag on the carriage includes "C.A.&T. Ferguson, London"
-Exactly how did the three cannons which were in London (two in V&A and mine) get from the Netherlands to the UK? The V&A seems to have acquired their two from a local business in 1887, but I haven't found anything on that business.