What size filter to buy for Canon FD 50mm 1.8 prime lens?

[AmauryfromBK]

I recently purchased a Canon FD 50mm 1.8 S.C lens. I wanted to buy a lens filter but unlike the new Canon lens, these FD lens do not have the size filter number on the lens. I bought a 52mm after doing minimal research but it does not fit because it is too small. The lens cap on the lens says its 55mm. I was wondering what size filter should i get, a 55mm or a 58mm filter?

Thank you