1941 to 1946
Would have been dated like this on the bottom. They made two version of each, mobilgas and mobilgas special. So a total of 4 versions. One has no outline around the horse and the other,like yours, had a blue outline. The did this for both mobil and mobil special.
Have to beware of repops as there are a ton of them. Small things can tip them off.
This is the first pump plate mobil used. The ones prior to that were decals. They also had a rectangle plate used around the same time frame.
Rectangle pump plate: