Hi Alexander,
You know I do not believe in complete lock plates before ca. 1500. To me, these illustrations rather seem to picture single divided lock parts like a serpentine spring just nailed or clamped to the right side of the stock!
I am attaching the famous snap matchlock gun of the Emperor Maximilian I and another two two ca. 1500 guns preserved in the Royal Armouries Leeds, one of which features exactly the same relic spring, the other having lost all its other 'lock' parts except for the brass clamps!
Please remember all these were preliminary stages prior to the combining lockplate, and that's exactly why I tend to date Martin Merz's drawing to ca. 1500.
Best,
Michael