It’s most likely a Cokin multi-image, or “kaleidoscope,” filter—specifically the Cokin 201 type. These filters use angled surfaces to split and repeat parts of the scene, creating multiple mirrored images around the frame. The center hole allows an undistorted central area while the shaped outer sections create the repeating effect. Cokin made these in different holder sizes (such as A and P), with the effect being the same and only the physical size differing. This type of filter appears to be discontinued, which is why it may be hard to find in current catalogs.