Thanks Alpha, I'm not sure which lens you're asking about so I took a shot of the setup. The cylinder is the Lensmate, which screws onto the camera around the fixed lens. On it's end are 49mm threads to take filters. The Tiffen conversion lenses are shown face down. They have 43mm threads just like on a filter. So I need to screw the conversion lens into the adapter just like a filter.

It's true that the conversion lenses are
smaller in diameter than the Lensmate adapter, but that's
the largest size that the conversion lenses come in. Brief
tests here show that only the telephoto converter has any
vignetting.