Just got this fairly nondescript 105MM shell that was fired in an M-137 Howitzer mounted in an AC-130 gunship. I cannot speak to the specifics of the gun, load data or projectile but am confident the case was base marked, as it was, for good reason.
The most interesting aspect is the letter that was inside the hull. A Memorandum For All Personnel from the Dept. of the Air Force. It notes that this type shell, along with the 40MM, 30MM and 25MM shell casings from AC-130 aircraft were henceforth not subject to restrictions against acquiring "War Tropies/Souvenirs" with specifics regarding demilling and certification(s) which included removal of the primer. This particular one has only had the center of the primer drilled out , luckily. The letter is signed by "certifying" EOD technicians on 30DEC06. The letter notes at the bottom that this is for a 105MM #1680, which is "etched" into the base.
My initial thoughts were I had paid too much, but after finding this little prize inside, it will stay in the collection for a while.