Otto, that looks like a Dr. Damper unit. The rubber can fail on those. Mine did, right away. They fixed it for free. Been fine since.

The stock Chevrolet ones look different. The damper itself is larger with the smaller pulley in behind closer to the engine.

1951 3800 1-ton
"Earning its keep from the get-go"
In the DITY Gallery
1962 261 (w/cam, Fenton headers, 2 carbs, MSD ign.), SM420 & Brown-Lipe 6231A 3spd aux. trans, stock axles & brakes. Owned since 1971.