I recently came into possession of this Gibson A-OO mandolin. I believe it is a mid 1930's instrument and the only information I see to help date it is the stamp on the inside of the f-hole which reads "1320-9". The 1320 is stamped and the -9 is hand written in faded ink (pen?). The pickguard, Gibson logo, basic ebony bridge (non-adjustable) and the tailpiece lead me to believe it's definitely post late 20's and before WWII. I am unable to see any other numbers/letters anywhere (and have a heck of a hard time seeing the end-block on the inside of the instrument). Any input is greatly appreciated.
Ignore the arm-rest. I added that recently as I tend to sweat a lot when playing on hot summer days.