Guess that those first coins with edge inscriptions were silver pieces too. Have not seen any but "my" encylopedia (Kahnt, Das große Münzlexikon) says that those first ones (1577) were thaler style issues from France. Apparently they were made using spring rings. For the later issues (1642 Clausthal/Harz, 1651 England, 1668 Denmark, 1670 Brandenburg and Sweden) a "Rändelwerk" was used - the coins rolled along/against metal bars and thus got their inscriptions.
Christian