I have 2 of the old grey boxes, I brought the micro box for about £ 5 or so years ago and i was given by a mate the bigger one. both still working fine. the have been dropped, left out when i have lost them for a weekend or so, run over with a quad, yet they still work! The only trouble i have is finding collars for them although i have been told that the mk3 collars work with them. Be warned they always seem to say the ferret is about 12"-18" deeper than they actually are! I have herd of a few ferrets killed my the spade whilst digging. Take it slow and use a spike to find the run.
A finder