The original Stingrays (and the Stingray Classics) had mutes on the bridge.
In the late 90's the mutes were removed (for potential injury liability reasons), but the bridge remained the same, so it was possible to buy the mutes separately, and fit them yourself.
Eventually the bridge was redesigned, and it was no longer possible to fit the original mutes.
3rd parties sell generic mute kits, that can be added to most basses - they're perfectly functional, but aren't part of the original instrument ; they're an optional modification / addition (like a drop tuner).
Rickenbacker 4001 / 4003 have mutes built in to the bridge (like the Stingray, and Sabre, they are applied to the underside of the string, at a pressure adjusted by grub screw).
Fenders originally came with a removable bridge cover, that housed a felt mute pad, muting the topside of the strings (they usually got removed, and lost). The Mustang originally had mutes, closer in design to the Stingray / Sabre.
Thank you! Very cool

