This old magazine article says "Krylon Dull Aluminum 1403"
http://www.atarimagazines.com/creative/v9n1/332_TRS80_strings.php
Before discovering that article, I myself have found "Dupli-Color Bumper Coating FB108 Medium Silver" to be very very close. The fine grainy look is just about perfect, maybe ever so slightly larger grain, but the biggest difference is the finish is slightly less glossy than the original. From 2 feet away you can't tell the difference.
That Krylon is still available, so I have ordered a can. I'll try to remember to update here when I finally try it. The article also suggests a clear coat, but Krylon themselves said they don't recommend any clear coat. The finish is probably already about the same shininess as the original, and clear coat would just make it glossier, but they were probably more concerned with protecting the high wear spots from wearing off again.