I posted this question on a Leica forum today and got a reply almost instantly, that I would like to share with you:
"Hello Toadmen,
Welcome to the Forum. You have a 2nd optical version, 1st mechanical version lens. A preferred combination. In 1962 some, but not all, lenses had begun to have their actual focal lengths engraved on the lens barrel. Many stated focal lengths of many lenses of many manufacturers (not just Leitz/Leica) are approximate. Engraving began @ Leitz around 1960 but was not universal until somewhat later. Lenses shorter than 50mm do not have their actual focal lengths engraved. The system of engraving was to write the LAST digit (ie: the "0" of "50") + the 1/10th of a millimeter of the ACTUAL focal length w/o the decimal .
Example:
50.0 = 00
51.3 = 13
49.7 = 97
Enjoy your lens.
Best Regards, Michael
"