I've got a Sigma Mini-Wide II. It's a very small lens - not quite a pancake lens, but small. think the Mini-Wide uses a 49mm or 52mm filter, definitely not a 62mm. So I think the Widemax must be a different lens than the Mini-Wide II.
I found a few things on Google by querying for
sigma +widemax, including an EBay auction for an M42 mount Widemax.
I like the Mini-wide pretty well - it's about as sharp as my K10 can handle, and I haven't noticed any over the top vignetting or distortion. I'm curious about what advantages the Widemax might have.