are you sure it is a gn?
Pretty sure. The handle rubber is a GN handle rubber and the stickers are obviously GN, but it might not be.
The shape is definitely not Hunts County as pacman suggested as I have one of each of those and it is neither.
On really close inspection, I can just about make out sticker shadow on the back, and it appears to line up with the GN checkers on the front.
The top of the shoulders each has a small round stamp on it. Z on one side and F on the other.There is also a tiny amount of writing on the top of the handle in ball pen with the two numbers "19" and "750". Finally, the inside grip is also a GN.
I thought the Optimum had a rounded back Jon? Any of the additional detail any use?