The page is correct. The standard (circulation issue) copper-nickel coin is KM# 671
and for a different metal version of it they add a letter at end, so it becomes KM# 671
a for (special collectors version) silver coin. There is also one member who has the silver proof in their collection.
https://en.numista.com/catalogue/pieces8954.html
https://en.numista.com/catalogue/pieces35935.html
Then if a coin was only issued in one metal, such as like you mentioned the 1,000 Escudos version below was only issued in silver, the KM number has no letters as there was not a copper-nickel version ...
https://en.numista.com/catalogue/pieces22294.html 
