There is a very long story behind this question But here is the short version.
S&W was purchased by Bangor Punta in 1965, with the help of a former Crosman Engineer the 77A was developed
After several moves back and forth from Florida to Massachusetts Bangor decided to sell the Airgun division to Daisy Airgun in 1980 but had dropped the 77A model in 1978. I don't know of any diagrams for the 77A but it is a forerunner of the Daisy Power-line models.
I would try to contact Daisy airgun to see if they "may" be able to help.
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