Well, I can't directly answer you question about value or other identifying info (sorry... I'm not well versed on this brand), but I'll try to give you a couple leads:
1. One of the best sources for determining current value is eBay. Search completed auctions for the brand & hopefully, something similar will show up.
2. I'm a little puzzled by the case-back, seems to be atypical of a fine quality Swiss brand, could it have been swapped for some rather "run of the mill" brand?? I don't know enough about B&M to say for sure, but it just doesn't look "right" for the quality level I know B&M had/has.
3. Other forums/groups:
a. Try Facebook if you are a member ... I belong to several watch collecting groups there. One (specializing in Seiko brand) has over 17,000 members!! So, you may find a large pool of knowledge there if they have a group that caters to that brand (or try a generic Swiss vintage group).
b. Timezone is another large traditional "forum" ... they may have an area especially for B&M, but if not, I know there is a Vintage area:http://forums.timezone.com
c. Try general web searches (Google, Yahoo) for vintage B&M ... I know that I've gotten valuable info for brands like IWC, Jaeger-LeCoultre, Omega, Cyma, Certina, and others doing that (including scans of vintage catalogs & sales brochures!). I've done that when researching items I was interested in buying & have often found helpful pictures/data that is good in ID'ing (checking for legitimacy and sometimes even finding a specific model) & valuing watches.
OK, best of luck!