Looks like a Unit Vehicle Transfer (Decal) for the Wessex Regiment. (Infantry) & possibly the Royal Wessex Yeomanry (Territorial Army Reserve) Like the US National Guard.
Unusual to see that as normaly boxes were not Unit Personalised like this. The TA are a lot more Leiniant with 'The Rules' that most regular Line Regiments! The Boxes themselves are not rare over here in the UKicon.