hi - welcome to the GMB

I believe that the C6 part of the code indicates 1958, and am only guessing but might suggest that the number 7687 could represent the factory pattern No. for this particular engraving design. Believe you're correct about the D standing for Ingeborg Lundin. I've had a look but am unable to see this particular engraved design in the Leslie Pina book 'Smoke & Ice'.
Hope you'll forgive my next suggestion, which is that if neither of you know anything about glass, then might it be sensible to suggest to your client that they contact someone who does. I say this from the point of view there might be something of real value which might slip through without being spotted. It might also involve a lot of time researching if glass is not your forte.
The word client implies a professional relationship, usually - but of course you may well be helping them
purely on a friendly basis.

Regret to say that we refrain usually from giving values, as they are very much a personal matter. Can only suggest that you look on sites such as ebay or failing which try some of the auction web sites.
A lack of knowledge of current values for Scandinavian glass might mean selling at less than the market value.
Of course if you have other items with which you'd like help on id, am sure we have folk here who will happy to assist.
Best of luck.