I agree that it was probably a screwdriver in its former life. The file marks were part of the forging/shaping process. They must have worked fast when they heated it up, to avoid burning the handle. Or, maybe they replaced the wood too.
I dug out my assortment of vintage Oyster knives. The one on the right up front and the two laying in back were made in Japan. The others are not marked (that I can see), aside from the second in from the left in the back row, which says Russell on the handle.
Oyster knives have a hard job. It takes a thin tip to slip inside and a bit of twisting to open them up enough to cut the muscle. So, they need to be slim, sharp and strong.
The second in from the right in the back row has a modified tip. It's been flattened off and made narrow; possibly for turning small screws.
Tom