Diamond Arms was one of the many house brand names used by Crescent Arms, which went out of business in 1932. Manufacture records have been lost, and dating is impossible.
I have a Diamond Arms Company 12 gauge single shot. The serial number is QT 585. My dad purchased it in 1923. I thought that he ordered it from a Montgomery Ward company catalog. Looking at some of the other postings make me believe it may have been Sears & Roebuck. In any case my shotgun should have been manufactured somewhere near or in 1923.