This is what ya want for the 505/C-D. Both have the square cylinder, meaning both have flat ports. This WILL NOT work for angled intake port cylinders (505/A-B).
Don't be lazy... get some sheet metal, make a riser and do it right. Some say you can get a bing on these if ya shave off the bolt and don't have a riser. I personally have about 5mm of riser and bolted a 15mm Amal up to mine.
Has to be mounted facing back, else port on the bottom of the intake will not line up with the cylinder.
Gotta flat port? (cough cough, square sachs cyl)? 
