I'm on the road for the week Uncleal so I do not have access to my library, however I do know that the ring sight you are noting in the photo was not particular to the Viper. The ring-and-ball was fitted to the Vickers as the primary aiming device for that weapon while the Aldis was meant to be used with the Lewis on the top wing. The accepted practice by many SE5a pilots was to use their Lewis for the longer shots as it had less bullet spread, while the Vickers was used when they were close in and had a fatter target in front of them. IIRC, the SE5a in the Shuttleworth Collection has both sights, just like the Vintage Aviator replica.
There were of course many field modifications made to the sights based on an individual pilot's preference.
It's the only only way,
It's the only trick to play;
He's the only Hun, you're the only Pup,
And he's only getting the wind right up.
So go on and do not stop
'Til his tail's damn near your prop;
If he only crashes this side in flames,
Well, only then they'll believe your claims.
So keep him tight in your Aldis sight,
It's the OOOOOOOOO-nly way!