They were 8rd single stack mags, you can find 10rd magazines, they were usually 9mm, and there was a few variants of the Mod 39 made. Like any S&W if carring a dash after the Model meant something like a change or mod. In the number series, a 2 number model is first gen, three digit model is 2nd gen, 4 digit model is 3rd gen.
Mod 39 could be either steel or alloy frame, steel slide
Mod 439 alloy blued frame/blued steel slide
Mod 539 blued carbon steel slide and frame
Mod 639 was full stainless
Prodction was over by about mid 80s
The Mod 39 led to the Mod 59, 459, 559 and 659 production, with 14rd high cap double stack in 9mm, then the rest is history