Purpose of side seam entry "pocket" on Bridge Coat?

I've recently acquired a Bridge Coat (size 38S, if anyone would like it!) and noticed that it has a side-entry "pocket" on the left-hand seam just under the arm (Marked by a piece of paper in the pictures!) This isn't a regular pocket, but merely an entrance to the interior of the coat; t's certainly intentional as the interior has a flap that can be buttoned shut to close it up from inside.

I'm puzzled by this--what could it have been used for? Would an item of equipment been attached to a lanyard that entered here?

Any help would be very welcome!

It is for a sword belt to be worn during formal ceremonies. The hook to attach the sword would stick out through this side entry. This pic is not of a Bridge Coat, but it's a good example how the sword is worn.

^ as Treetopflyer says... incidentally, you also see this on British Guards overcoats, although they don't have a nice little flap like on this bridge coat.

Excellent--thank you very much for this, treetopflyer and DD!