Also the J-45 had a baseball bat style neck (instead of the J-35's "V" style neck, used on pre-1939 models), and a tear drop pickguard.Sunburst was the only finish available for the J-45 (or most any war-time Gibson guitar, as a sunburst finish can hide wood flaws, an important feature during WW2 when wood supplies were short).
At the time, they were not an expensive or fancy guitar. ) If I was stranded on a desert island with dollars to my name (the price when introduced) and could have only one Gibson flattop, a 1940s or 1950s J-45 would be it!
Just knowing what I know, I've tried to assign some years to these FONs. (Though any "1941 J-45" does seem to really be a 1942 model by all other standards.) Remember in 1945 Gibson didn't seem to use FON numbers on J-45's, so there aren't any listed below.
9xx FONs to 2xxx FONs red spruce tops - often lightly built, usually lighter than the 7xxx H examples and sound warmer but not as powerful with beautiful harmonics.
This style of tuner lasts into early 1943 (FON 2221 last documented series with this tuner style). (Late 1942 FON would be 2119, with FON 2143 being the last documented series with mahogany neck block.) Some FONs for 1942 include 7116-7119, 7434, 7705, 7721, 907, 910, 923, 928, 2004-2006, 2059, 2098, 2110, 2119.
Bracing for 1942 to 1944 J-45 models (9xx FONs to 2xxx FONs red spruce tops) was often lightly built with full sound.
This is located inside the body's sound hole on the neck block. Joe's book lists a lot of FON numbers for 1940-1945, but unfortunately doesn't largely specify exact years during this period.