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On my Speed Booster (for Leica R lenses), the locking pin readily goes "click" to hold the lens in place. That said, the fit of the lens is very tight--after only a moderate number of lens changes, there's already significant brassing on the mount--so fit and finish on some of these things isn't as good as one would expect from a premium adapter.
That said, I'd recommend getting a vernier caliper, which can be had at pretty much any hardware store here in the US. Cost is about the same as a pint of beer, so not a large outlay compared to the cost of your photo gear. With it, you could measure the outer diameter of the pin that sticks out of the lens mount on the Speed Booster, and you could measure the inner diameter of the channel it is supposed to fit into.
One possibility is that the lens has a non-Contax mount--that someone adapted it to Nikon or Pentax or what have you. (A visual indicator of this will be that there is more than about a millimeter of nickel metal showing from the side of the lens mount. I've never handled a C/Y lens, though, so compare yours to internet images.) Of course, if the mount has been changed, it won't work with a Contax-mount Speed Booster.
Hope this helps!