Man, this brings back memories! My first job right out of college was with Firestone. I was an office/credit manager, but had to assemble bicycles...LOTS OF BICYCLES, in my downtime. Huffy made most of the brand labeled Firestone bicycles, and, at least after 65', there were no Firestone 500 bikes with the large tank you describe. In fact, I don't recall ANY Firestone 500 bikes. There were regular bikes with the two flags, usually on the front fork. I believe that the one you have is at least from the 50's...maby before.

ENJOY!

Russ