At first sight I was also inclined to think that this was a modification made by a skilled amateur, who perhaps owned an old straight grip pistol and wanted to bring it up to date soon after Webley introduced their new slant grip design. However, it then struck me that the grip is removable via the two securing bolts (unless I am misreading the pictures). Surely an amateur would have no reason to do this, when brazing would be a much simpler and more effective way of securing the grip to the cylinder? On the other hand, if Webley were contemplating changing the rake of their straight grip pistol it would make a lot of sense to adapt a straight grip pistol in this way so that a variety of grip designs could be tested without having to go to the expense of forging a complete cylinder-grip unit each time.
Another point to consider is that changing the rake of the grip frame of a straight grip pistol could have been achieved in various ways without having to discard the old trigger guard. In this case the new grip frame has an included new trigger guard, which would be an unnecessary and major engineering hurdle for an amateur but not for Webley.

So on balance I think this has a good chance of being a genuine Webley prototype, and could be of historical significance.