nq,
The moon is actually a lightly stamped Bomb shell.
The B means this receiver was made by Singer Manufacturing Company (not Singer Sewing)
Singer made and supplied the bare receiver to Underwood.
Adds some value to it.
I see your Recoil plate is a post war cast type. If mine I'd replace with one marked for Underwood.... that closely (If possible) matches the receiver in color.
Thx for sharing it.