It's a a replacement pickguard made by WD. I was just on their site and thought this might look cool.
WD® Custom Pickguard For Fender® 1978 Musicmaster Bass®
MM's were made with Mustang bodies and spare parts to my understanding. Those included strat pickups. So this one has been replaced.
In my humble opinion, 99.75% of the time black on black is the worst combination.
I've only seen it work with shiny new bodies, and the white on the 3 ply made red.
Although, I love tort on black, you may want something not so "done to death".
I have a black/tort Axcelerator which is beautiful, but this is what I did to my MM.
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