Tom D said
I believe Andrew has a trench gun receiver that has had the barrel replaced as well as the buttstock.  The serial number puts the received right in the trench gun serial number range.  Also, the hand stamped “US” and Ordnance bomb marks are characteristic of the trench guns.  Most of the riot guns were in an earlier serial number range and don’t have any US markings on them. 

I don’t believe the hand stamped marks were applied during arsenal repair because I have seen too many WWI trench guns with those marks that are in excellent condition.   One was even an unissued trench gun in 99% condition.

My data shows that most of the WWI hand stamped “US” and Ordnance bomb marked trench guns are in the 670,xxx serial number range.  Then few show up with the hand stamped markings until the serial number range of Andrew’s shotgun where there are several marked that way.  

Wow, guys – many thanks for all the responses! ?

I agree with Tom D that my gun started out as a trench gun, but has been ‘converted’ with a different barrel and stock.

Here are some of my notes from reading both the 1992 and 2007 editions of Canfield:

On the WW I marked vs unmarked trench guns: “The guns that were actually issued before the end of the war would likely have been inspected and “passed” by ordinance inspectors prior to acceptance into service, hence the added markings. The guns that remained in packing crates after the war were not issued, thus there was no need to stamp them with martial markings.” (Canfield, pg 52, 2007 edition).  

“Third, a cache of 74 WW I production Model 1897 trench guns that came to light in 2004 sheds a bit more light on the subject. The Richmond (Virginia) Police Department received these trench guns circa 1924 from the Virginia National Guard (via the U.S. Army). The Richmond PD retained the guns for the next 80+ years in essentially the same configuration in which they had left the factory. While these guns were unquestionably made under military contract, they were never actually issued to Army units during WW I, hence the lack of martial markings.” (Canfield, pg 53, 2007 edition).

On the trench gun markings vs riot gun markings: “Most of the confirmed original WW I riot guns were stamped with a small ‘U.S.’ on the left side of the receiver and a very small ‘flaming bomb’ on the barrel and also by the serial number on the receiver. Unlike the the martial markings found on some WW I-vintage M1897 trench guns, the riot guns marked appear to have been factory stamped prior to final heat treating of the metal.” (Canfield, pg 55, 2007 edition).   Canfield’s pictures of riot gun marking on page 46 of the 1992 edition and page 55 of the 2007 edition show that those are quite different from trench gun markings.

By the way, the 2007 edition has twelve pages of “Collector Notes, World War I  Combat Shotguns” (pages 49-60), quite a bit more than the 1992 edition.

Anyway, I am still having great fun with this research.  I have sent a note to the Phoenix Police Museum to see if they have any records on my gun.  Not expecting much, but who knows. . .

What do people think is the story of the stock?  Clearly not WW I.  Looks like WW II stock, complete with sling swivel (with “factory quality” inletting).  But none of the proper WW II markings/cartouches.  I saw something in the 1992 edition about post-war refinishing removing those markings, but were they so shallow that a refinishing would have completely removed all trace (as is the case with my stock).   Hmmm!