That shim well not stop peening on a brass frame recoil shield. It's the ratchet hitting the recoil shield that causes peening. The spacer in the photo is for keeping spent caps from jamming the revolver. These spacers are from the Manhattan revolvers if my memory is not to far gone.
Keep your barrel gap tight, .008 or so, and keep the load under 18grs for a .36 and 22grs for a .44 brasser and you'll be OK.