I didn't catch the longer brl as stated in your original posting. The longer brl and serial number sounds like a Model 1919 in a sporter stock. I doubt that it came that way from the factory.
Pull the butt plate and see what serial is on the butt plate and stock. A lot of the time the stock and action don't match as there was not much fitting requires but might help identify the time period of the wood.
As for the dual firing pin bolt, it was only used on the 1919 and 1922 with serials before 25,000. (Both models had separate serial numbering and both started over at model change at 25,000)
The opened port area on your rifle would be correct for the serial. The smaller port was only on very early 1919's and maybe some 1922's (I have not seen one).
