Nor have I, but I just spoke with Keith at the Custom Shop, who celebrated his 40th year with Remington last Friday. He believes some "D"-grade Comps were built but cannot recall how many might have been produced and records for that era are no longer available. He does recall the Custom Shop embellishing an 870 Comp receiver every now and then before sending them to the production line for rollmarking and assembly.
From that information, it's apparent that any high-grade Comps were not true Custom Shop guns as they not assembled there, but we have learned that the triggers in those guns were assembled with extra care and removable-barrel shotguns aren't subjected to the fitting tolerances of a bolt-action rifle or break-open shotgun, so that might be a moot point.
It's safe to say that if just 5,300 Comps were ever built, and that seems to be an accepted figure, there sure couldn't been many high-grade ones and those guns, if still in nice shape and unaltered, should command a respectable price.
Ed