Congratulations on what sounds like a nice vintage Model 336.
I am unfamiliar with "Paco Kelly's date calculator"* and I have no answer concerning the "solid lever" description. I do recall seeing subtly differing Model 336 lever configurations. (This having nothing to do with straight grip vs pistol grip lever configurations.) I just thumbed rapidly through Doug Murray's book on the Model 336 and I don't see the lever exemplars there. Perhaps it was in Col. Brophy's book on Marlin rifles, which I don't have in front of me. I have provided a few photos below of my "G" SN prefix "Waffle Top" model 336A rifle including one showing the action & lever a bit closer. The "G" prefix does signify 1950 production and that was the introductory year of the 35 Remington chambering. I believe that the waffle top configuration shown in a photo below was transitioned out during 1953. As to the lever situation, unless your rifle presents some evidence suggesting a replacement lever, it seems highly unlikely that it is not original. I too would like to have a few original vintage Marlin sight hoods! I have more than a few Models 336 and none have the hoods.
Largely from memory...
*A Website address reference specific to the calculator would be appreciated.