Sheridan 20 Cal.

https://discover.crosman.com/blog/benjamin-sheridan-product-dates-of-manufacture

This should get you started. 

If this link does not work google "sheridan serial numbers age"

Benjamin & Sheridan Product Dates of Manufacture (crosman.com)

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.

On original RACINE Sheridans the "YEAR" will show up on the LEFT SIDE of receiver @ under the loading tray STAMPED IN and be in REVERSE.

Thus as a 1966 year gun will have 6691 etc ....

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.