Duly noted, Jim.
Not that the 'extra' letter i thought i dscerned (a A ), would be unmatching to your considering the marks provenance.
Yet on the other hand, the Castel Sant'Angelo rack number hypothesis you are digging into, doesn't sound plausible to me; usually a rack number refers to a row and line in the racks, and not to the floor where the rack is kept. For such case, you would have to have three letters (digits) in the marks, in case these were placed in more than one floor which, as you know, only the 4th floor was used for the purpose. However this is quite a riddle per se. It is no easy job to distinguish the concept between armoury (remnants) kept there from early ages, some now exhibited in noawadays museum, and that put up in much later times, when the castel became a classic barracks; many centuries in between.
But to keep your flame burning, i can tell you that Pius IX 'crack corp' of foreigners was comprised of 4.592 men, including 135 Canadians and 14 Americans, the so called Zouaves; as if one of those brought your sword to the Americas.
Just en passant and, as you probably know, Castel Sant'Angelo (originally Hadrian’s tomb) is not situated in(side) the Vatican, but near the Tiber River 2,600 feet from the Vatican city. Pope Nicholas III had the brilliant idea to build the famous Passeto di Borgo, an hidden elevated covered corridor, through where the Popes and their guard would flee to the castle when things became hot. I have visited he Castel/Museum in 2010, but didn't access the corridor
.