I believe they are solid brass, and not brass plated. Brass plating is rather fragile, and would not have survived intact; with all the battle damage those cuffs show. In particular, the file marks Joe Tanner refers to would certainly have penetrated to the base metal.
The cuff casting and the pins are different brass alloys. The cuffs look like yellow brass, while the pins look like red brass. There are probably good manufacturing reasons for that. If the cuffs were plated, then the differences would not show.
I see what may be the remnants of plating around the keyhole, and on the pad beneath the steel locking pin. In the latter case, it may be a trick of the light.
My guess is that they were plated, the plating started to peel, and they were stripped.