There are more recent examples like the pobjoy mint £1 isle of man coins in the early 1980's at the ideal home exhibitions they had a special privy mark on them.
It might be interesting to get a list of exibitors from the great exhibition , to be honest i cannot see the royal mint being there but the heaton mint or moore could have been .
I am asuming the new model crown came after the much more common gothic example produced in 1848 or a little later