hi - the numbers to which you refer are possibly what are called 'matching Nos.', and not as might be thought, the dates.
Most modern decanters of some quality, are fit matched at the factory, and both the stopper and body of a pair are given the same No., showing that they corresponded, in terms of fitting, before leaving the factory.
Nos. can be anywhere from 1 to well over 100, and are seen commonly on C20 decanters, although it can sometimes be difficult to find them on the glass, and for the stopper they seem mostly to be on the bottom of the stubb as you have commented. If you look carefully, you should find the same No. somewhere around the lip, neck (inside and outside) of your bottles.
A small and seemingly insignificant feature, but one which has great use in showing whether stopper and bottle are the same pair that left the factory, when considering buying such items.
Presumably your bottles have a Dartington backstamp or is there a label ??
John is usually better at these things, but until he looks in I'll take a shot at suggesting these are FT (Frank Thrower) 15............ or at least some variation on that particular design. The stopper isn't exactly what I would have expected, but the factory, for whatever reasons, did change stopper design more than once I believe.
It's a ship's decanter shape as you probably know, and assuming it is FT 15, then this design started life originally some time in 1967 - BUT, that doesn't mean yours are that vintage. These two may well be much more recent, which would account for the variation in stopper. Colour wise, as well as clear like yours, this design was also offered, originally, in midnight and kingfisher.
Let's see what others think.