[Morrow] 5BR Knobs?

[Colburn]

I have one 5BR and one 5BR-1.  The knobs on the 5BR are black plastic or
phenolic, on the 5BR-1 they are metal.  Is this correct per the
originals, please?

Also, what is the best way to clean the metal knobs?  Given the
experience of folks destroying the dials I don't want to accidentally
damage the knobs using a cleaner that removes some sensitive finish!

At this point the plan is to clean these up, assess their as-original
status, and sell them to help pay for the restoration of my MB-560.
 
73, doc kd4e