It sounds like the specific part you're referring to is the metal bracket that houses the "no mar contact tip" and is integral to the depth set mechanism of your Ryobi Cordless Brad Nailer P320 (2016). Unfortunately, without a specific part number or a clear description, finding this part can be challenging.
One suggestion is to reach out directly to Ryobi customer support. They may be able to provide assistance in identifying the part and possibly ordering a replacement for you.