Nice find, yes that box.
I am going to have to sort out a timeline for British Shermans. The Osprey Firefly book (by David Fletcher) only mentions the hybrid almost as a throwaway line on page 46 after mentioning M4 shortage in August 1944. I know there is a difference between production and delivery dates, and the Firefly conversion is going to set them back further. The Oliver book lists only two Polish ICs in 21st Army Group in June 1944.
I have found a IC hybrid with a Firefly box on the front from 7th AD, probably B squadron, Vth Inniskilling Dragoon Guards (NVG 141 page 33).
I have just identified the X76 tank, 153rd (Leicester Yeomanry) RA, Guards Tank Division AOP. Page 17 in the Dennis Oliver book. I missed that in the BTIN book (page 37).