Many thanks Darrell.
When I was at the Camaro Nationals in Frederick, MD this summer I saw a survivor L78 which appears to have the nipple on the water pump that was unpainted. I'm not at my home computer now but will post a picture of it tonight.
I'm also trying to unravel which type of nipple was used on both the intake manifold and water pump. Unfortunately the '68 AIM does not list the part numbers for a BB. It would, however, appear from your picture and also the car that I saw at the Camaro Nationals, that the water pump nipple used a long hex type fitting and the intake manifold used a short type fitting. Not surprisingly all the Camaro vendors websites I visited have what appear to be the wrong type hex nipple for the big block water pump nipple. My car is an L35 but I would suspect that the L34, L35 and L78 all used the same type nipples but as you say, let's see what other say.