J56xx numbers would be mid-late 1950's
(The numbers go up over time)
>Gm used J numbered tools , among others
Kent Moore 's office was in the GM building , they had a rather close relationship;P
Many of the tools KM made for other cars also ended up being tools for GM cars because GM bought 1/2 of the car makers in existance....
The round crowsfoot says Aviation to me....