[Citroënbender]

Chain tensioners of fifty years ago, often used to have a mesh filter screen - wouldn’t be surprised if this detail is long gone.

Excepting the polymer tensioner shoe, the rest of a unit can usually be dismantled and solvent cleaned off the car to restore function. Ones with a strong spring element (that moves quite a bit) may suffer ageing of this part, but most designs are simply to take up progressive slack - not large fluctuations like a serpentine belt tensioner.

Always worth familiarising oneself with how to retract and then release pre-tension in hydraulic units, at least these are a simple grenade pin type.