The bike is very obviously a Yamaha TT/XT500 (with mods).
These case-savers were made and marketed by Pro-Tec (USA) during the 80's. They are made of about 3mm Aluminium Alloy and shaped with the intention of safely guiding a broken chain through the countershaft area without bunching up. Their internal radius runs fairly close to the outer diameter of the chain's rollers - so ideally they should be sized to suit the sprocket you are running.
For anyone that has thrown a chain on the Yam 500 motor, these things will save you having to repair the oil return pipe after the chain gives it a good whipping!
Pro-Tec also made guards/guides that were like steel strip material wrapped around the outside of the chain. Pro-Tec also made lots of stuff for the 500 single - where are they now?
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