The Olympus 35 RC camera has a fairly fast lens (f/2.8). This means that when using 400 ISO film, it can be difficult to expose correctly in bright light without the use of a neutral density filter. The problem with the 35 RC is that the filter size is the difficult to find 43.5mm.
The solution is to use a step up ring to a regular filter size but where do you find 43.5mm setp up ring? Well, at CameraFilters.com they have a great selection of filters and step up rings. I went with the 43.5mm to 46mm step up ring and then use a Hoya 46mm ND (x2) filter.
CameraFilters.com also sell a lens cap for 43.5mm filter sizes so if your 35 RC is missing it's cap, this is the place to go.