Ok here are the results after a Rust911 soak.
There appears to be the remains of a (now) gray color plating on the "cap" portion only. It might look a bit like corrosion, so I scraped a small area clean, and there is no corrosion underneath.
Sure doesn't look like nickel but does look like a coating/plating of some other type. Thanks for taking the time and sharing
Looks like some of the areas in some of the examples I posted on page 1 of this discussion
I held back from posting the examples below
since we all realize that service replacements can have different finishes and look (as well as the whole picture/monitor things) but here are three examples. The upper left is a blue and white early label NOS/service example from likely 65-67 and the other three are later service parts from the late 80's or much later. To me it appears that a phosphate or similar treatment would likely reproduce a similar look depending on the lighting. In some it appears zinc phosphate while in other pictures magnesium phosphate would be a better choice