That doesn't mean it was Army issue, in fact it may be that you are misreading it and it really reads armory steel, just hype. The name Diamond has been used on a number of firearms, however Diamond Arms Co. was a " Trade name " used on firearms sold by Shapleigh Hardware Co , St. Louis Mo. It could have been made by a number of different manufactures who specialized in making Trake name guns. , Crescent, Davis, Meriden, Stevens just to name a few of the possible US makers and it is even possible that it was imported from Belgium ( look for proof marks under the fore arm ). The most likely U.S. Manufacturer was Crescent Arms Co. However , the name, Diamond Arms Co., was owned by Shapleigh Hardware and they could have used it on any shotgun they sold. Since they purchased their shotguns from the lowest bidder, they more than likly changed suppliers several times. I know one collector claims they are Steven Shotguns,. Who knows"? These were inexpensive shotguns when new ( read cheap ) and time has not increased their value. You gave no indication of condition nor pictures so only a WAG can be given to value. In the conditon most of the old shot guns can be found, I would guess maybe 50 dollars or less. There no valid records of when it was made, a guess would be in the 1930's to early 1940's. I can tell from your post that you were holping that it would be a rare Army issue shotgun, but alas, it was not meant to be. There are still treasures in the attic, but this is not one.