I think this tank belongs to the Cadman brothers, however if I'm wrong. I'm sure Ade will know. I must say the add ventilation holes must be a big bonus in the hot July weather which normally blesses the war and peace show. As to the what made them a hollow charge anti tank round, so take your pick from PIAT practice up to the more recent LAW 80. Plus some of these older "Range relics" were used for Panzerfaust evaluation and testing during WWII. As to the condition as Ade will tell you I have a bit of thing about restoring stuff, so if this Grant tank was mine, I'd be blazing away with the Arc welder and welding up all those nasty holes with stainless steel rod as part of restoration of this tank back to factory fresh condition.
BTW does anyone know what engine this tank is running on, its it the range original GM Detroit Diesel 6-71 setup or a replacement unit.