Can't find underlined capital F costume jewelry mark please help

[bassfishing11497]

Have a vintage (I'd guess 1940s from the way its made) pin/brooch marked with a capital letter F with an underscore beneath.  Do you know who the maker is?  Photos of mark on back of piece attached. Checked illusion jewels with no luck.

[bojibelle]

Could it possibly be a poorly struck block E for Eisenberg?

[bassfishing11497]

Wow Eisenberg jewelry is beautiful.  I read where they used a capital block style E logo around 1960 which would fit in with the date of this piece.  The only E logo though I saw didn't look quite like this mark.  But the style seems to match pretty well.  Thank you for your thoughts.  Maybe it's someone unknown.