Some extra source material:

• the first beeing a door panel from the "schutterij gilde" with two target guns and a striking iron.
• the second a facing brick from the tower of the Amsterdam "kloveniers tower". Dateable to the second half of the 16th century, showing a heavy arquebuse with smaller foot rest than the later 17th century pieces. Dificult to tell for sure if this is a evolutionary step within the design or just an exception. The corners are both flanked by bird claws on a ball, the city sign of the "Amsterdam kloverniers".
• the third picture is a rare insight in the shooting hall of the "Amsterdam kloveniers" as it would have been in the late 17th century (print by Jacob Appel.
• The fourth is a Penning, a type of currency or commemorative coin. In this case from the Middelburg shooting guild (the Confrerie van de edele Busse > The guild of the noble guns). Dated 1623, similar to the one in post 30 though very hard to see due to the small details imprinted on this coin. On a side note, these type of coins were also awarded to the members of the guild and could be exchanged for a pint of wine. Other examples have the Guild mark pon one side and a funerary bier on the other side.

This lock showes two matchlock/tinder lock mechanism on one lockplate. The left one is a standard trigger bar activated lock, the right one a snap tinder lock with a primive tumbler (red) with a spring for the counter clockwise action of the tinder holder. The tumbler is held in place with the dual acting sear. How this would work puzzles me a bit, the picture is of too low quality to decern the mechanics around the tumbler and sear. My guess it rest against the sear (yellow) as a square surface?
This lock does give some insight in how the missing lock on my gun could have looked like. The idea of a tumbler seems for now the best option.