In 1963 my part of the world (Dog Patch, USA) the HB M-10 had just became available. .38spl was the un-official police caliber. The .38 S&W cartridge was the chosen caliber for older B/T pocket revolver. At this era many surplus M&P was flooding the country from England.
It is possible that a "Special Order" was made for Grandfather. However the .38 S&W cylinder could have been from surplus factory parts as was the RB, but the HB would be a mystery as a barrel blank would have to be special bored for the slightly larger bullet ?