I've seen a lens called NIKON NIKKOR 50 mm 1: 2?

On my of these auction sites I've seen these manual focus lenses, ai, ai-s or non-ai

and often only mention 1:2 and not the aperture. I think it's an f/1.8 lens but it confuses me. Does 1:2 have something to do with magnification?

The "2" in "1:2" should refer to the f number. And back in the seventies Nikon did produce a 50mm f/2 lens.

F/2. The fact that 1.8 wasn't in that number is a good indications that it isn't f/1.8. FYI, whenever you see numbers expressed in a ratio form, they refer to the aperture. Lens manufacturers have had many ways of expressing the maximum aperture of a lens over the years, but it is most often known as "speed". The lower the number behind the colon, the faster it is (relatively). The thing that will really confuse you is magnification, which is expressed similarly.

There's a manual focus nikon lens --NIKKOR 50mm f/1.2,… Could be it.