The UWL-100 comes in several flavours and INON data seems to indicate it is designed for 28mm equivalent lenses there is one footnote saying only compatible on 14-42 lens. Here is their page: http://www.inon.jp/products/lens/uwl100achromat/top.html
The website indicates using a type 2 lens for olympus cameras. I believe also the lens is not designed to zoom through only to use at it about the widest setting close to 28mm equivalent on the lens: " Set the camera to wide end. If you observe vignetting at wide end, zoom to telephoto side minimum necessary or crop the image after taking image. The lens is designed to get optimum performance at above setting. "
Before I went out and a purchased a flip mount I would suggest just getting a step down ring to m67 and screw it on to give it a try and see how it performs, you will probably need to zoom into about 14mm on the 12-50.