Georg, good point about the insistence on having the open sights available in addition to the telescope. It's debatable whether there was any benefit to having the open sights as well: would the rare instance where the sniper found it necessary to use them outweigh the inconvenience and difficulty of the offset design? McBride mentions finding it useful to be able to use both on the Ross at times, but the Ross had a fully adjustable aperture backsight, rather than the open sights of the Mauser or the SMLE. Hesketh Prichard mentions an engagement he organized where the offset telescope on the SMLE caused a pair of snipers to be unable to get onto the target in time.
The Germans learned the lesson sooner than the War Office obviously, and the CanadianCorps went so far as to discard the back and foresight on the Ross entirely when the A5 scope was fitted. Presumably done to allow the scope to be mounted lower over the action.
Your photos reveal the origin of those base blocks as I'm sure you realize.
Another odd thing about the Whitehead fitting is that they bring the telescope ocular much farther back than most of WWI scope fittings. Looking at your photo from the previous page,
the scope could even be pulled back another 1/2 inch or so depending on where the locking ring was set. For a prone soldier that's going to be at least an inch into his eye socket. So instead of that rearward 'dog-leg' on the rear base, if they had made it curve in the other direction, they could have mounted the front base on the side of the backsight 'block' and had a perfectly stable setup!
One of the few advantages of a target scope like the Winchester models was the ease with which eye-relief could be adjusted, and the locking ring set to whatever position one wanted to return the scope to upon pulling it back to the firing position; done with springs on later target scopes of course.