I'm looking for some cheap lenses for my father's STX-1.

I've tried to school myself, but nothing make sense and I've even found conflicting informations.

>X fujinon
>Fujica X
>X fujinar T
>Porst

I know for a fact X fujinon lenses fit, some of the Porst also do. Fujica X should be the actual name of the mount.
The problem is that it's impossible to search coherently among the sea of dumb labeling, since no one on ebay mentions the correct mount.

I'm considering a 135mm 2.8, I found a few of which I believe the X fujinar T should not fit. And then a Porst, which should fit. Am I correct?

Is there any other lens that might be of interest?
I'd like something with a good aperture since I'm gonna be doing some night shots but I'm aware old lenses are not Ideal for it.