Omega Dynamic Chronograph 1998

Check that it is running properly. These have the same servicing complications as the Speedmaster Reduced. That is to say that a large proportion of independent watchmakers wont touch them. If it has problems, you might find that an expensive trip to the official service centre is the only course of action guaranteed to get it working well.

I speak from personal experience, mine was problematic.

I don't think it signifies anything really. It might have been a quality control stamp either for the maker of the caseback, a dealer or perhaps as you say it might be a watchmaker mark but I wouldn't read too much into it. There are often lots of strange marks on casebacks which usually have meaning only to the dealer or watchmaker who put them there. This watch is as you say nearly 25 years old so the presence or otherwise of a historic servicing mark wouldn't sway my decision to buy in either direction. You want to see service proof from within maybe the last 6 years, or else plan for a bill sooner rather than later and set the price you think is acceptable accordingly.