Just a bit of info on these if I may....You will find that one end is 22mm the other is 18mm...in case you want an anti glare tube to fit. The other thing is that when I turn mine around you have to loosen the bubble as it is to tight to push through and turn, therefor you would have to adjust bubble again. after turning. Whether mine is just a one off I don't know, it would be interesting to find out.....Having said all that, when turning from 50 to 100 I am around half inch from centre. and vice versa.....so it works.