It's perfectly legal to have it here ONLY if you have the Prohibited Bore firearms license (Equivalent to Class 3). People here usually have it on the Non-Prohibited Bore license as the LE here have no idea what it is
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The one on my Glock is the original FSSG switch and Ive shot in excess of 10,000 rounds without a single hiccup.
There are a few chinese variants available but they're crap.
If it would'nt have been for collection's sake I would have never bought the Glock 18. With the switch any 17/19 runs exactly the same as the 18.