Definitely a piece to hold onto and keep in the family.
For reference, the Model 39 series evolved from US military testing during the 1950s intended to replace the aging M1911A1 .45 caliber pistolsd, with no such conclusion actually reached.
Colt submitted their lightweight Commander (a shortened version of the .45 service pistol) in 9mm caliber while Smith & Wesson submitted their new 9mm pistol that later became the Model 39 series. The Model 39 design included features of the Walther P-38 pistol adopted as German Army standard in 1938, very similar double-action and single-action modes, hammer-drop safety, with the dust cover slide design and Browning-style pivoting barrel locking mechanism familiar from the M1911-series.
The Model 39 was adopted for standard issue by several police departments, with the Illinois State Patrol among the first major contracts. Probably the first significant uses of semi-auto pistols in North American law enforcement.
By the mid-1970s S&W designers completed the Model 59 design, incorporating a thicker grip-frame to contain a dual-column single-feed magazine very similar to the Browning Hi-Power type. Evolution of the design continued with stainless steel and compact versions, double-action only models, and many other options (659, 469, 669, 5900-series, 6900-series, 3913, multiple others) and related projects in .40 S&W, 10mm, and .45ACP calibers.
The pistol shown in the OP was probably manufactured in the 1970s, a fairly early example of the Model 39 series.. The black rubber Pachmayr grips were popular with law enforcement users during that time period.