First: Very good idea to collect info on these modified limited editions, sold as seconds. I am planning to write a short note on this topic for Scotland's Glass at some stage - and it would be useful to see as many examples as possible for this purpose, to see whether there is a "trend" in how this happened with time.
You yourself have mentioned the
1975 Dragonfly, also a limited edition.
I had mentioned an ebay auction of a 1974 Sea Crab, also a limited edition.
There were also images of a
1970 Moonflower - now, this is an unlimited edition. This raises the question: why was this Moonflower modified in a similar manner as "limited seconds", as unlimited weights could be sold as seconds without such a modification? For me the most plausible explanation would be that someone just practised on them.
I am pressty sure I have seen similar discussions on such modified Caithness weights on this board before: if you have one such weight (or had one) - would you, please, speak up by either linking to the already existing thread, or posting your images here.
Second: What happened to weights which didn't meet the standard? I guess we have to differentiate: there will haven been weights with minor flaws (like some change in colour or a small unwanted air bubble somewhere) and others where just something has gone wrong altogether (plus many grades in between). I am sure the really bad ones will actually have been destroyed - but the ones with minor flaws were sold in the factory shops. This was no problem with unlimited designs - but with limited designs something had to be changed to make it a "different" design, in order not to exceed the edition size. Now - as mentioned before, different people will have different opinions about this procedure

- but as collectors we'll have to live with it.
Finally there is the question of marking seconds. There are scratched marks "CIIG" and "CG2" (and possibly others) and sandblasted marks "CIIG" - permanent marks, although they can be polished off, of course. At some stage it was decided, however, just to put "unselected" stickers on the base - which tend to "fall off"

. Fortunately at this stage weights (first quality) were marked on the base as Caithness, usually with the design name added - whereas in the first years the was no marking (for unlimited weights) on the base - just a sticker (which again may have come off or been taken away on purpose).
So to distinguish:
1. Early weights will be marked as seconds (unless first quality).
2. Later weights may just be not marked at all (when a second).