Pre 57 Benjamin 312

I'm throwing this out there as I am relatively new to classic pumpers, having only rebuilt one blue streak. I was given a pre 1957 Benji 312. I come to this conclusion because there is no serial number on it. Furthermore it has a straight bolt handle with no knob on the end. Based on the history of this gun from the second owner I believe it's a 47 or 48.

Two questions. 1. Can anyone date this gun with what I have described?

2. The black oxide is worn off, and the metal under it appears to be nickle, not brass. Should I leave the patina alone and just reseal? Or re-coat with a blackening product? Suggestions on the blackening product would be appreciated.

Please Help...Im open to thoughts and or suggestions.

Squelch.
 
A little more info. The only date codes I have ever been able to find,which I'm sure you have already found, is here.
http://www.crosman.com/discover/crosman/benjamin-product-dates

Strangely though, one of my Benjamin rifles is a model 312 and has a serial # of H21586. I purchased if from the original owner who claims it was purchased new in 1953. It must have been manufactured before the Benjamin company started keeping track. It has the one piece curved bolt like most models have.