There are two split image screens. The 4 degree prism is the most general purpose (No. 2 for the C330/f, B for the C330s), while the 6 degree one is optimized for shorter lenses (No. 3 for the C330/f, or B2 for the C330s).
Most 35mm cameras with interchangeable screens did the same thing. The prism is sensitive to the divergence of the image light, and they usually fail around f5.6 as well. Microprisms are less sensitive to angular effect but also fail when the aperture drops.