Did anybody
ever throughout the 60ies and 70ies play a long scale six string bass tuned B to E i.e. C to F? I'd be surprised. There was zilch market for it and any experienced luthier would have frowned at the sound implications of the string tension on the high B and E i.e. high C and F. You cant get those to "sing" if they are thick enough to still sound like a bass and if you thin them out they sound like an overstretched guitar.
If there was a dedicated long scale six-string bassist somewhere, then Gibson might have built this as a one-off for him
if he was famous enough, but where is he then? I can't think of anybody. None of the 8-string bassists I know from that time ever dabbled with a 6-string.
That Gibson would say "Let's add a B and E (or C and F) string to our new long scale bass to hear what it sounds like!" seems incongruous to me given how both the 6-string EB-2 and the 6-string EB-3 had flopped even as niche models in the 60ies. If that is how they approached prototypes, no wonder they were technically broke by the end of the decade.
PS: There is something iffy with the truss rod btw too - looks like they added a spacer below the nut. String pull might be an issue, but I've seen modified 8-string RDs hold up to the additional tension without such "help".