An interesting scope combo.

[Lee Enfield]

It's funny how you don't immediately see what's staring you in the face....

Looking at the image of the prototype No74 scope, it's obvious that the mounting system discussed earlier is exactly what they originally envisioned.

It's not possible to see how this has been constructed dur the limitations of the image, but it appears to be much more robust mounting base.

Perhaps a separate prototype dust cover?

I assume the bean counters got involved and insisted on a mount assembled to a standard top cover, probably dooming it to failure...

The prototype illustrated appears to be mounted on a modified X8E2 (Ex2) unit sight top cover.
https://www.forgottenweapons.com/x8e1-x8e2-slr-manual/
The extended scope base, addition of the locking tabs to the rear and deletion of the front locating pin.