Does anyone know what unit the flashes on this WWII British Grant tank represent? I think it is a unit of the 7th Armoured Division, but I cannot figure out what it is (as indicated in the "40" flash).

These number marking are the unit code, they were different between units and on different colored squares.

 

In example. Check ot this page for the different ones found.

 

It's a long page so make sure you scroll down to the bottom to see it all.

 

https://www.flamesofwar.com/Default.aspx?tabid=110&art_id=590

One more for other Divisions.