I looked into purchasing one of these a few months ago (for my m1 in festival settings).
Seems like I read it would power the m1 for approx. 10 hours (as stated above).
When I went to order one from the site listed in this thread, I backed out, because the shipping alone cost about half as much as the battery from what I recall. If you could find one local it would be a good deal. I opted for 4 rechargeable ratshack double A's, which will give me 10 hours use as well, and not add bulk to my gear bag. The AA rechargeables have gotten better & better.