Is it possible there are pieces missing? Looks like trays may have rested on those ledger boards front and back. No idea what that would mean but they are there for some purpose.
I believe the pieces you are seeing are simply there to hold the center divider in place, without having to cut a rabbet in the front and back.
(HistoricBore, you can see that I can fit 7 shells if they are alternating. Not so if they are all heads or tails up.)
There are also screws on the front and back faces to keep the divider from falling out:
[url=https://ibb.co/LvYbKsG]]
Is it big enough to have once held a brace of pistols? Perhaps there are sign of other partitions in it at one time?
I see no signs of other partitions and I don't think it's nearly big enough to hold pistols, unless they were very small.