Re: Posts #2 and #6
In my haste to be helpful, I may have unintenionally confused the issue. BPs began with BH-1 in 1938. BH-657 thru BH-1549 were mfg in 1941; therefore your machine base was mfg in 1941. Now for a few sites:
www.lathes.co.uk Specs, everything--M heads are on p. #2 of Milling, Drilling, and Boring heads--bottom of the page. Great site for specific info.
www.google.com Google "bridgeport mill M head manual [123 pp].
www.practicalmachinist.com Mother Lode of information on head conversions and just about all you would want to know about a BP--beware--thousands of posts from the finest experts in the machine tool world. As an aside, the general consensus on that site states the diffulculty in determining which base came with what head. Please use their search window for M Head specifics. Sincerely hope this clarifies my original post.