I have an actual Canon lens mount convertor P which is very simple so, as excalibur2 notes, you may not be able to use non-automatic M42 lenses, and I've only ever tried it with preset lenses myself. However, by the look of it it might actuate the stop down pin so automatic lenses can be used in stopped down mode. Unfortunately, I don't have any M42 automatic diaphragm lenses in my collection to try it out. The convertor is breech block mounting on the body side and very well made. They seem to sell for about £20.