Your rifle can be aged based on its serial number. Please see the link below.
What is the age of a Sheridan c series 5mm 20cal rifle serial number 370862
Sheridan air rifles who is owned by Crosman Sheridans are 5mm air rifles
At one time Sheridan, Benjamin and Crosman were all separate companies. In the 1970's. Benjamin bought out Sheridan and took over the company. In the 1990's Crosman bought out Benjamin. Now Crosman owns all 3 companies. When Benjamin bought out Sheridan they decided to keep the C series design. So they decided to keep the Sheridan name on the .20 mm air rifle and put the Benjamin name on the .177 and .22 caliber 392 series air rifle that looks just like a Sheridan C Series. So All Sheridan C series are .20 caliber and the .177 and .22 are Benjamin's. Crosman decided that this worked well and kept the same arrangement for both models. If you go to the Crosman home page you will find (At the bottom of the page) a link to (Owners and Parts Manuals) there you will find the owners manual in the Benjamin / Sheridan section.
It was manufactured in 1978
Crosman owns the Sheridan line of air guns. They offer a service to locate shops that repair older products. See this link. http://www.crosman.com/general/find-a-service-center
Sometime around 1980. In 1977 Benjamin bought out Sheridan and for a time they merged the names Benjamin/Sheridan.
1974 see the link below
I believe you are asking about a Sheridan .20 cal (5mm) air rifle. They started to make this rifle in 1949 and stopped in 1990 ( sheridan sliverstreak model ) During the 1990's the name changed to C9 sliverstreak and C9 bluestreak. The value is determined by the current condition of the rifle. You did not give any such information , therefor I can not answer you.
It depends on the model, but usually it is right on the barrel or the receiver. If it is really old it may not have a serial number. For a time Benjamin and Sheridan did not serialise their rifles.
Crosman air gun company owns Sheridan. Crosman offers a free service to locate shops that repair Crosman products. See the link below.
You have given me all the information except the model. Sheridan has several different models at different prices
There are 3 versions of the Sheridan Blue Streak. See the link below for the 2nd version. BTW today Crosman owns the Sheridan and Benjamin line of air guns. This link is from Crosman