So, I find this other Line Seeker for auction at eBay this evening....
http://www.ebay.com/itm/Graftek-vint...item5892e7ee6c
If the description from the seller is correct, It seems like I'm on the right track with the history of my bike, as I found the same on my frame and in the internet so far.
Unfortunately, the example for sale have a seatpost clamp ear missing, supposedly cut off on purpose, according to the seller.
Could it be Exxon demanding back then that all Graftek prototypes/predecessors be eliminated to clean up the Graftek introduction history before buying the rights to the Line Seeker design??...or it could also just be bad brazing/welding at the seatpost clamp ear to the seat lug too......

Chombi