I'm not quite sure about his date of manufacture. That list has always confounded me. There are three things about the description that are not accurate. His is not the first variant. The first variant has the old tootsie roll pump handle. I have several. The serial number usually begins with a "T". I have some that also do not begin with a "T". His does not have the checkered stock and fore end, and I have both styles, smooth like his and checkered.
Please understand I'm only pointing out things I have encountered over the last 25 years of collecting Benjamin air guns.
I also have several Benjamins that are pre-serial number which dates them to before 1957.
The green box of the two I show is pre 1957. They are very hard to find.