I'm a bit confused about what you're asking, but it may be just faulty memory. The original housing for the 1st GoPro has a round front domed port that resulted in slightly blurry U/W video unless some adaptations were made or an aftermarket housing used. I seem to recall GoPro did eventually come out with a second housing that fixed the problem, but it was so long after many other fixes were marketed, that I didn't pay much attention. If I recall, it had a square front flat port. Which do you have?
Most of the fixes to the original round port housing resulted in a standard round threaded front port that would take any suitably sized photo filter, such as a URPro or similar red filter. If that's where your rig is, you just need to determine the diameter of the port threads and either buy a filter of that size, or a larger one and use a step-up adapter ring. There have been round threaded U/W filters of various diameters around since the film camera era, so you have some array of choices of size.
I don't recall what the external filter solution for the unmodified housing was, if any were ever offered. You can always cut a gel filter to size and stuff it inside the port if you like.
