http://tinyurl.com/cewvtf
That's a volume adjustment control from a 5302. Although I've not ever seen one installed on a 302, it's relatively simple installation.
I don't have a 5302 yet, I didn't realize they had volume switches. That's gotta be a Frankenphone. It was un-retrofitted.
Some have them, some don't. But, that is a factory job. I've also seen them on standard 302s. Normal? Heck I don't know??