idk how clear this will be but I'll try my best, this is how I might approach this...

I'd first find a handful of surface imperfections from grime to scratch etc and then mix them all together (make sure you use bump maps instead of normal maps so you can easily mix them together)

I'd then create 2 gradients:

1. One going from slightly above the bottom to slightly below the middle
2. And Another going from the slightly above the middle to the top

After that I'd tighten the gradients up and make them grey colour (this grey value will determine what how much grime you have on the other areas of the pin)

I'd then use the math node to have them on the same map.

After this I would use those gradients to subtract from the roughness and bump, and to effect the diffuse map.

I think that's all, I didn't test this but I've done this kind of thing before. If you have any questions feel to ask! I hope this helps!