I have been told that there is one other of these dark blue FCG Co wax sealers in Half Gallon.  There are a few others in a lighter cobalt and this is one of the more common cobalts in quart (although still not in any way common).  I have one of the lighter cobalts but was thrilled when this one turned up.

It's got what appears at first glance to be a bad repair on the back of the lip but after looking at it carefully, I'm convinced that it's actually original to the making of the jar.  Others may disagree, but I think there's good evidence to back me up here.

FCG Co stands for "Falls City Glass Company".  I believe that they were located in Oklahoma.  Cobalt was used as a color in these jars on the theory that light caused the contents to go bad.  The cobalt was supposed to negate those effects.  However, it was really bad in the "Cobalt blue makes the food inside look disgusting" problem.  There was quite a bit of experimenting around with cobalt and black (which was colored for the same reasons) in the 1880s or so.  In the end, it seems that people just couldn't handle the change and these colors dropped quickly out of favor.