Thankyou!
You know, some time ago I stumbled upon
this page, as I remember seeing it before, where the X-Nite CC1 filter is specifically discussed. But when specifically searching for camera filters, could I find it again? No!
There's also
this page. Easily found if you search on the right terms (X-Nite CC1)!
In summary, the
58mm X-Nite CC1 filter is exactly the right one.