I have a gadget bought via eBay a few years back that allows you to use Cokin filters on my C220 and other Mamiya TLR's. This device holds the filter over the viewing lens, then after adjustment it swings into position over the taking lens.
Before obtaining this, I used to hand hold grad nd filters over the viewing lens, note its approximate position, then hold over the taking lens, it was a bit hit and miss, plus you needed the camera on a tripod, no bad thing. I also used to use a polarising filter in a step down ring on the viewing lens, after adjustment note its position by the writing on the side, then transfer it to the taking lens. Using yellow, orange, red or opaque IR filters is straight forward, simply screw into the taking lens. The TLR comes into its own when shooting B&W IR film. With the opaque IR filter, R72 or similar, screwed to the taking lens, you have a clear view through the viewing lens to compose. Note that the chrome and black lenses have different size filter threads. Great cameras, I have the the 55mm, 80mm and 135mm Sekor lenses.
