Sorry...user error.
It was not the locking screw, but the spring loaded pins on the device.
Maybe the Canons have a more gradual slope or something.?
Anyway, the last row of pins was not being pushed all the way up into the wireless unit, and that was prohibiting getting it the last 1mm into the shoe.
Afraid it was just a case of Beginner/Inexperience on my part. Did not see it until i put the camera upside-down, and took a look with a decent light.
Thanks Guys