Coins, paper money, or exonumia of which the obverse and/or reverse
design features an identical
design from another coin, piece of paper money or exonumia of a different denomination.
"This Decimal Day
2021 50p has all the old pre-decimal coins on it, is there a term for coins like this?"
"Yes, idenumia coins, banknotes, etc have design(s) of other numismatic item(s) on it.
Like how you'd use the word 'piedfort' to describe a coin that's
double the regular thickness."