Originally Posted by
dook
The instrument in the illustration shows a white line limit of M1.7.
This is commensurate with the limitation of Lightning Marks F1, F1A F2 due to directional stability and radome strength considerations.
If you're referring to the photos in post #5, they are from the rebuild of F.2A XN728's cockpit, hence the limit line.
Here's an original in situ:
But we're still left with the mystery of the OP's photo.