Nieuport fighters, types from 23 to 27

As far as I know, the above named Nieuport fighters had the Vickers gun
fixed in a slightly offset position.
As that was not the case with the Ni. 17, I wonder why this choice was done.

Any suggestion ?

Thanks in advance.

The N23 had a revised construction that required the gun mount to be offset due to internal changes to the structure. It also had a revised airfoil and other minor changes. Externally similar to the N17 the only real way to visually tell differance in photos is the offset Machine gun. In RFC N23s you couldn't tell differance because they had Foster mount Guns like the Nie.17.