In 1955 was probably lucky to have a 100 amp panel and if a load test was performed today your panel would most likely fail the test my guess again hard to tell from here
You could use the date system you suggested as that is what several manufacturers use (A-K) 1-12, but there is the possibility it may be original.
You may want to contact these guys: FPE replacement, and they may tell you for sure.
More than likely, it was originally fuses and only had a 60 amp service, no central air, and a gas water heater and range.
When the central air was added, they upgraded the electric to, hopefully, 150 amps and changed to breakers. I base that on the physical size of the door in your picture, as the 100 amp FPE panels were much smaller. I can’t make out everything on the label.
Judging by the manner in which the panel was labeled, I wouldn’t be too sure as to who installed it. A permit search may be helpful.