This is a .38 Military & Police Airweight, an alloy framed version of the famed .38 Military & Police Pre-Model 10. When the Korean War started in 1950, the US Air Force put out a call for lightweight, compact personal defense weapons they could issue to aircrews and this model was used as one of the iterations of the M13 Aircrewman and simply had its markings changed.  Unfortunately, the alimunum alloy that was so cutting edge and lightweight that it helped Smith & Wesson win the Air Force contract was this product’s ultimate undoing. Many of the aluminum cylindars and frames of these revovlers showed significant breaks, cracks, and weaknesses, some units having ruptured altogether. For the civilian marketed M&P Airweight, Smith & Wesson discontinued the use of an alloy cylinder after producing about 120,000 and used steel instead. In using their M13 Aircrewan iteration, the Air Force found that the revolvers required low-pressure M41 Special Ball ammunition as they couldn’t withstand commercial ammo loaded with lead bullets.