In the old days when I used to do that for my advertising studio prints, we had a custom stamp made by a local typesetting company and used a regular ink stamp pad. This was, of course, for fiber-based prints.
The information would individually pertain to your line of photography. Obviously name, address, copyright, etc. Date, negative file number or commercail job number. Whatever would facillitate finding the negative to make reprint orders.
RC paper requires a special spirit-based quick-dry ink. It is touchy to work with. The PPofA magazine used to carry classified ads for this sort of thing. Portrait people often make a lot of money from re-orders.