12/75 build date, and it looks to be about 600 before the last 76 MY rolled off the line at 4762903801 - since there were only about 4400 76's.
More correctly than a "Custom Color" - the #98 paint code indicated a special order paint color from one of the other 911/912E paint colors offered that model year, but not offered "stock" on the 914s.
Whereas, paint code #99 was a Special order to customer request or color match to any other color not offered that model year on any other Porsches, including prior year's colors of Porsches that were no longer offered.
Black and all Metallic Colors on 914s were also an additional charge "Special Color".
It looks like the color my buddies 76 911 was - that Copper Brown Metallic you found someone mentioned on the other post of your 2 914s. I've never seen it on a 914, and it looks pretty nice IMHO, and I'm not even a brown car guy myself!
No one else here has seen it on a 914 so far (as the OE factory paint), so it could be a one of a kind color choice by the original buyer.