You are right, Spiridonov,
I attach two detailed pictures which I took of that gun when it was on display at the Military Museum in Prague in 1995; you can see that the rectanguar pan around the touch hole on the top flat is a later repair, a working addition of ca. 1500, the once swiveling cover is missing.
The length of 73.8 cm must be the total length as the length of the bore is hardly ever measured.
The other barrel seen above the bronze gun seems to be somewhat earlier at first sight, ca. 1440-50, but the touch hole is already on the right side - maybe this, too, was a later working alteration and the original touch hole on the top side was nailed and shut as has been the case with many barrels. This is mere speculation, though.
Best,
Michael