The F stamp near the serial number on the water table of the frame is the grade of this gun. It is not unusual to see a Lefever that has barrels with a higher grade stamp than the frame. But I have never seen a Lefever with original barrels that had a lower letter grade stamp than the frame.
I find it a bit unusual that the 9641 serial number barrels had that 9641 number on both barrels. When we see Lefever barrels with a second serial number, the original serial number is on one barrel, and the added number is typically on the other barrel. I cannot recall ever seeing a Lefever with the serial number on the barrel flats, such as you describe on the 26" set. So I tend to believe that the 26" barrels were fitted to this gun after it was built, and most likely the work was not done by Lefever Arms Co.
Also, I have seen shortened Lefever barrels that still touched at the muzzles. You cannot simply look at the muzzles and judge if they have been shortened or not. You need to examine all of the evidence such as the length of the choke tapers, the degree of choke, the position and centering of the sight bead, the presence or absence of keels, the solder, and the overall workmanship and condition of the muzzles.
