A small magnet. Hole drilled in aluminium barrel cover and magnet Araldited in place. Have done my mk2 as well, a slightly different approach though.

https://imgur.com/a/qGx2X

Never liked the ball detent latch so thought I'd have a bash at something different. <blockquote class="imgur-embed-pub" lang="en" data-id="a/nPr5M"><a href="//imgur.com/nPr5M">Mk3 TX200 with modified underlever latch</a></blockquote><script async src="//s.imgur.com/min/embed.js" charset="utf-8"></s...

A few pics of my mk2,

https://imgur.com/a/qGx2X

I've got a MK2 which was bought second hand. Previous owner had modified the barrel weight so a 1/2" unf male threaded silencer can be screwed on. Cam't remember who I had it off but I'd say anyone with a lathe would be able to modify it.

These seem cheap http://jsb-diabolos.de/?field,28&sLang=en

Not used them personally as i still have a good supply.

Best way is to set out paper targets every 5 yards to cover the range you shoot over. Then zero your scope at 25 yards (or whatever suits you) and if you have resettable turrets set the top turret to zero. Then all you have to do is shoot at the rest of the targets one at a time adjusting top turret...