If there is no binding and the receiver was properly masked to maintain tolerances, the inside portion of the locks show no signs of plating, and the lock-to-wood fit shows no cramping... Then it's likely a job done in conjunction with Hunter Arms.
I say this because I was informed by the old gentlemen at Ithaca that it was a common practice to plate these guns and a firm in Rochester did the job for Hunter.
Now... The question being whether we should consider this "factory" or whether it should be considered an alteration. My feeling is that it seems Hunter would deliver a plated gun to a client if asked. Just how it was looked upon by Hunter remains unclear to me, but the guns were obviously constructed and finished with the plating process in mind.
Which leads me to conclude that I wouldn't hesitate to purchase a plated gun... Bearing in mind the fit and finish would have to be of a level unlikely to be matched by anything but a professional organization operating in close cooperation with the manufacturer.
Sorry not to have asked more questions twenty years ago as the sources have long since departed this life. It just wasn't an issue then and now I wish I'd have had the good sense to pry more information out of the Ithaca Custom Shop.