A well seasoned frog it is. Somewhat unusual marking...don't think I have ever encountered this particular manufacturer before. A. Carter mentions this maker with verified examples from 1940 & 1941. He also referred to this form as BBF #310. However, the cross-strap for #310 is sandwiched between the front & rear sections of the frog body and retained by three lines of stitching. Your example has slots for the strap which look rather crude, so this could be a conversion (#312).