The design of these might make them very useful for some amps that have built-in bias meters. I ordered a couple to try in my Scott LK-150.
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With bracket: http://www.ebay.com/itm/260427077781
Without bracket: http://www.ebay.com/itm/261234005058
No affiliation
With bracket: http://www.ebay.com/itm/260427077781
Without bracket: http://www.ebay.com/itm/261234005058