The ROF(F) one I have also has "39" inscribed with an engraving pencil on the side where the extractor screw head is. When I measured it; sure enough, it's .639. I don't remember seeing any other bolt head marked this way with a "scratchy pen". Y'all notice that the length doesn't at all coincide with what ADE's very well done chart says either.
Roger, that Maltby No.4T is lovely.