This is a common chassis which provides the usual -50v / 0v telecom power and can house different LTU.

I have two variants of these, both rack mounted in a 3U chassis, both are made by Telspec. I believe in normal use, there would be a two pronged jumper to link vertically, and for testing these could be removed and inserted horizontally to loop back.

As with the other pics I have, I would appreciate any feedback as to where they would have been used and what determines which LTU is used for which purpose.

This one seems to have extra monitoring available and a loopback switch to help testing:-

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