I searched for some time before 1) figuring out the filter system of the C lens SWC and 2) finding the right filters and adapter ring. I had to send a few filters back to their sellers in the process. Beware, most sellers don't have a clue and advertise "filters for SWC", which is often not the case. What you need is the following:
- a 63 mm unthreaded filter,
- a retaining ring.
I am not aware of any adapter for the old C-style SWC lenses. I don't think one ever exists, unfortunately. Due to this I limited myself to a clear protection filter (my SWC goes pretty much everywhere with me, and I am a bit paranoid), and an orange filter.