Hello Ian. Quality decanters made throughout most of the C20, often have matching Nos. inscribed on both of the stopper and the flange or neck of the bottle, and most of these Nos. are made using a steel stylus of sorts and can often be difficult to find.
It is just possible that the mark on your stopper is one of the pair of these matching Nos. - and if you look around the neck/top of your bottle you may find the other one. Sometimes these Nos. appear to have been applied with little care, and they aren't always easy to decypher.
Lots of lesser quality bottles are found without these matching Nos. and moulded examples also lack this feature - plus Nos. tend to disappear as you get back into the C19.
Have to say I don't recognize the cut pattern on your decanter - assuming you haven't worked it out by later today, then I'll try the Replacements.com site