If I had one I had posted it, it is in a class case that the BAM will open for nobody. But from what I can see the spring must be located under the sear, just like on a wheellock mechanism. Your editing thought was pefectly right!
I attach images of the similarly construed lock of my Tusco-Emilian (Brescia) snap-tinderlock arquebus of ca. 1525-30, the serpentine shaped as a seahorse.