Gents,
A small detail I forgot to consider, my spares book is for '54 to '57 bikes, which explains much of my ignorance. However, Bill, I wonder if the "8" in "280" could not actually be a "6". On my own rocker box, said number is almost impossible to read under the number stamped over it. A very good photo I have of the backside of a part on ebay looks more like a "6" than and "8". Then, going to the online stores, 67-0280 is listed as a rocker box stud. Apologies for getting too much info from the web, versus experience with multiple bikes (does taking the same one apart repeatedly count the same as having more than one?), but one of the online distributors in question is SRM, so I am fairly confident in the information.
All this being said, I don't think I've contributed much to the question of stamped numbers versus cast numbers. I know that valves and springs changed between '54 and '57, so it makes sense that machining of the rocker box may have as well. I think it also makes sense that in the earliest models there was no need to stamp over the cast-in number. The question is, if Brian is looking for rocker box, and it only has 67-259 or 260 cast in it, with no over-stamping, can he assume it will fit a later model? Without more detail and, maybe, a direct comparison with a known correct part, and based on my limited experience, my answer to that question would be, "no, he cannot assume."
Richard