Your explanation in cutting the entire rib out of bar stock is much more sound than my shaky sheathed approach. I don't defend my position as much as I cannot explain the back portion w/exposed brass.

I even thought perhaps the thin sheetmetal an illusion until I felt it, whilst looking through the 10X loupe holding a LED light, w/point of a razor knife.

On that one millimeter twixt the back of the rib and the frame there is a lateral cut in the brass. This cut is not clean and could be post installation exploratory gash. That screams hand held Dremel cut-off wheel post-installation.

What would be the effect of blue on brass? Nothing?

It still could be solid steel. They could have cut a channel in the back of the rib and filled that w/brass rod and no flux. Maybe that would mimic a cast appearance. As well the top front of the two piece base surrounding the blade sight each have what amounts to a single file short stroke at 45* to the top corners. Again, perhaps exploratory by a previous owner. No other reason for that not to be blued as well and it is a shiny steel.

Except for being a softer metal that is easier to melt and conducive to use in molds there isn't much reason to use that application when equipment/skill is readily available to tool steel.