The BB shell on Thorn bikes is SS, if you were so inclined there's no harm in stripping the paint from the numbers, or engraving them alongside. There's talk about re-establishing them, but they haven't gone anywhere, they've just filled with paint. For the same reason powder coat will spoil the look of a frame with fancy lugs. I'd assume Thorn's frame builders used the same stamps for their DuPont painted frames and they are much easier to read.