Can't answer your questions about "worth," but I will offer this -
According to ColtFever <Diamondback>, the Diamondback was produced from 1966 to 1991.
Yours (1968) is certainly a comparitively "early" one, being built in only the third year of production.
The question of which type grips your gun should be wearing is one that should be answered first and is dependant on when Colt made the transition between the early "smiley" (First Type Target Grips) and the "straight line" (Second Type Target Grips).
I don't know the answer, but I'll bet someone will come along that does.