We've had something like that in our window above the Tax Disc for years   

Coincidence that this has appeared as the Tax Disc is being phased out!  Wonder if you could get pulled if it was spotted by an over zealous bobby for not showing the correct details as TD's will be about for a while yet.

The Tax Disc has gone, you now don't have to display one since 1st October, hence the use of this empty space on your front windscreen