Meta l/ Mineral Test Piece

Cleaning out the shed today and found my old metal mineral test piece that i made from iron filings back in 1970.
    I would find the position on the disc control of my vlfs  where the detector neither responded nor ignored the tester and I would use that setting to test individual rocks for metal content above the level of the test piece. Worked quite well.
    In the All Met GB mode I found the GB position that very very slightly responded positive to the tester and used that setting the same way I used the disc mode setting. encased it in epoxy resign so as not to break it.  Worked quit well .... Still does.

Minelab was not the first.  

I guess it is like the ferrite used to set up the GPZ....Only mine is better and cost almost nothing and I thunk of it before ML existed., although I don't actually recall when ML came into existence. I think possibly that Garrett had a similar method of setting up the metal mineral test mode function on the Scorpion??