Were produced in the 1970s. There were two models the 77g and 78g only difference was the caliber of the pellet. They resemble a Model 41, 22 cal target pistol. Usually, no matter how nice they are when you find them the seals are likely rotted from time. I had a supplier that listed the rebuild kits for these but haven't needed a kit in 20 years. Somebody must still make them. The value hasn't changed much, in the box in excellent condition about $100. Resealing it is a pain, so do it if you have the skills, the time and the patience. I found this see here: Air guns - Pyramyd Air Report: Smith & Wesson 78G and 79G - Part 1
PS: the original blue S&W CO2 cartridges and S&W tins of pellets have some lesser collectors value.