David,
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Make your own, buy either a three point gauge in 00 finescale or EM.
Make yourself a few inches of 14mm dead straight track gauged with callipers.
Cut the bought gauge in half, and take a few mm out of the centre. Place one part on each rail of your standard track. Apply Araldite to the top surfaces. (File flat first if there is any raised lettering). Put a piece of metal, almost anything will do, on the Araldite. Weight on top of the rails so the whole lot goes flat, or clamp, hair grips, plate and gauge sections. The other way is to build it upside down with the track rail down on top.
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Go to bed, Araldite will have set by morning.
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I'll be offering one of those as well, if I can find it!
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Frank