It is alternative name listed in Standard catalogue of world coins.
1701-1800 6th edition gives following ratios for monetary system til 1825
20 Reis = 1 Vinten
100 Reis = 1 Tostao
480 Reis = 1 Cruzado
1600 Reis = Escudo
6400 Reis = 1 Peca
It does not list pataco in ratio table, but shows pataco name as alternative for 40 Reis on individual coins.

I assume Portuguese might have more insights on it.