If you still have some of the lower Al content stock, work up a part or two (coupons) for fine polish, and also work up a part or two or three of the 0.004% Al current stocks, and polish these up also in precisely the same way, same duration, etc.
Put a hickey on each part, or a stamp mark, that allows you and only you to distinguish these parts.
Next, put all the parts in front of the customer, and have them pick out the ones with poor polished characteristics, if they can.
Do not be surprised at all if they get about 50% of the high Al stock parts picked out in the "good" pile, and put about 50% of the really low Al stock parts in the "bad" pile.
Then all you need to do is inform them of the results.