This is what I found in doing some searches.... (I lead a boring life...)
From one site: (http://en.allexperts.com/q/Guns-Firearms-Projectile-1501/2008/5/value-gun.htm)
National Arms was the house brand name for a company called H&D Folsom Arms Company. The specific house brand name was used for firearms manufactured for May Hardware in Washington, DC and for Moskowitz and Herbach Co. in Philadelphia. (this info courtesy of Sid Lark, another expert on this site).
H&D Folsom was not a manufacturer, but a distributor. The actual manufacturer was a company called Crescent Firearms which later became Crescent-Davis firearms. Crescent was controlled by Folsom until around 1932. Crescent manufactured guns for hundreds of retailers, distributors and mail order houses. Almost all remaining guns from this manufacturer are priced as shooters and have no collectable value.