What is this, and how should it be adjusted?
 

This is on a '95 Trek 930 I just picked up - Screwed to braze-ons under the chainstay.
All I can think of, is that it's an anti chain suck device.
If so, it the first I've seen that came as original equipment, instead of a later add on, to correct a problem.

Anyone know how it's best adjusted?

anti chain suck plate .. should just clear all 3 chainrings .. of those 3 sizes ...

Thank you - Those are the sizes of the oem chainwheels on the bike, so it's all good :)