Interesting that it's fillet brazed. I also see what appears to be Agrati dropouts and right hand threads on the drive side of the shell. Consequently, the 'D' stamp almost certainly indicates the right hand side of the shell and could be threaded either Italian (destra = right) or French (droit = right). However, if the Reynold is imperial size and the dropouts are Agrati, then the source is likely Italian.