I have no idea as I have never owned any Benjamin, Sheridan or Benjamin-Sheridan pumpers, so I don't have a "dog" so to speak, but if I were to make an unscientific WAG (just to see how close I can get with NO knowledge of the gun), I would say mid-late 1960s to early 1970s.
Actually, that is my guess and I only posted this so I could more easily find this thread later!
So... when you find out how old it actually is, I can see how far off I was with my UWAG! (smile)
Good luck and let us know when you find out, that is, if nobody here can date it.
Wow, I remember as a little kid thinking how a Sheridan was top of the line. My dad finally bought me a genuine Sears 22 caliber pump up and I slew a lot of birds with it that I regret now.