Claude,
The best that I can surmise is that the lock is upside down to take advantage of the fact that heat and fire will rise when the pan charge ignites. I suspect that the pan was shaped to channel the flash into the vent rather than losing much of benefit of the flash to the normal upward trend.
Still seems like you are exposing the entire lock to a firey bath of bp fouling with every shot. Also note the area just foreward of the lock. Seems to have not enjoyed the fire bath on that wood. The builder may have anticipated some downward damage with what appears to be a metal plate or trigger bolster. I wonder if this was part of the original design or a reactive measure.
But then, maybe the builder was hoping to test exactly all of these thoughts with this piece and continued to learn through continued modification and testing. The fact that we never see anything like this anywhere else likely tells us the result.
Robin, you sure do find some interesting items. It is a pleasure having you out there providing such amusement to the ML communities.
YMHS,
CrackStock