I believe it's Stainless Steel; I need to chemically polish the slot
in the head which is not accessible otherwise.

Big problem is that electropolishing doesn't like to penetrate down
holes, and is very good at rounding off sharp edges like you probably
have at the edge of your slot. You don't get significant polishing past
about 10-20% of the diameter down a hole, and deeper than that (down to
maybe 50% of the diameter) you can actually roughen and degrade the
surface as the current density falls off below what gives good
polishing. Don't know where you live but most any big city will have a
few electroplating/electropolishing shops. Take your part for a visit
and let a pro look at it in person.

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Regards,
Carl Ijames carl.ijames at verizon.net