Lugernut, that question was answered in the post several down "Interesting Markings on a 1913 DWM". It stands for Bohler Steel and the number signifys the hardness. But look for the explaination by "Garfield". I hate to admit it, but it's pretty good.
Welcome to the Luger Forum.
<img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />