Wow. I made that drawing of the Fessenden many years ago, but I'm afraid I don't remember much about her.
The guys in charge of the Coast Guard web site are generally pretty good about looking up the information that's available about those old ships. The big problem is that the old Revenue Cutter Service was notoriously lousy about keeping records. My guess is that the material on the website is all the folks in DC have been able to dig up.
About the only other suggestion I can offer is to look up the ship in Donald Canney's book, U.S. Coast Guard and Revenue Cutters, 1790-1935. Don Canney is a thorough, conscientious researcher; I remember when he was working on that book, and I'm confident that he dug up what there was to dig. If the data on the armament isn't in that book, it probably hasn't been found - and isn't likely to be.
Sorry to be so unhelpful. Good luck.
Youth, talent, hard work, and enthusiasm are no match for old age and treachery.