I used to have a standard S&W 39. Nice little gun; no "match pistol" but it would digest almost anything that was stuffed in the mag.
Speaking of the mag; it appears to have been a direct lift from the Walther P-38 magazine, with the only significant difference being the addition of a 1911-style mag catch slot. A P-38 mag will plug straight in, but not be retained in a '39. Never owned a P-38 so never tried things the other way. S&W probably saved a LOT of R&D money by "borrowing" the basic design of the P-38 mag and grip
That looks like it was the prototype for the Navy Seals Hushpuppy. S&W worked with them on it and the first stainless steel model 59 was a result. It was also what lead to the 147 gr. 9mm sub sonic load for the Hushpuppy. Seals used them to take out sentries and dogs that the VC used. Then they would enter a VC encampment and do what they had to do. S&W later brought out the 639 and the 659 stainless steel 9mm's.
The 147 gr. bullet was never meant for use by Police or personal defense. It was only for use with a suppressor.