I don't think they were ever shiny, in the same way as paint is shiny, even new. Pictures seem to show that the rubber bumpers had a matte finish, with maybe a little shine to them. I've seen restored B's and Midgets with shiny waxed rubber bumpers, but I seem to recall that getting them that way is a lot of work, with sanding, buffing, and waxing. Not to say what those products might do to the rubber itself. The bumpers were never painted in period (althought BL toyed with the idea), because there were no paints available that would remain pliable enough on soft bumpers without peeling or cracking, especially in the cold. I seem to recall that the only one they found had a toxicity level that was unacceptable for use on passenger cars.
-William