Hi Dabourg.......welcome to TFF.
Your shotgun was made between 1917 and 1932.
Davis-Warner Arms Corp. was established in 1917 when N.R. Davis & Sons bought out the Warner Arms Co.
In 1930 Davis-Warner went out of business, but Crescent Arms bought the rights to the name and briefly assembled shotguns under the name (probably from parts acquired in the purchase) until Crescent was, in turn, purchased by J.C. Stevens in 1932.
I don't see a "Diana" listed, but for a typical N.R. Davis & Sons single shot, in average condition, figure $100-150.