Looks interesting the Webley adjustable silencer. As a HW99 owner myself the problem regarding the adjustable silencer is that the 99 barrel is not spherical so it wont adjust/tighten over the sight grooves, it would probably throw the silencer out of line, you would have to file the flanges down to get a snug fit, if you do that you may as well get a better make and push on fit.
I looked at this problem when I first got my HW99 a few years ago, the correct HW silencer is ugly and expensive, I backed out of altering my barrel from standard by filing the grooves out so I just learnt to live with it as it is,s its not all that noisy and don't think about it now.