These are for holding ready-use shells - but a very specific kind that was safe to store this way.
The British 120mm from this period fired separate ammunition - a shell was entirely separate from the powder can.
File:Australian naval gunners with 4.7 inch ammunition 1944 AWM 016472.jpeg - Wikipedia, the free encyclopedia These angle brackets held the shells used for ready-use and thereby simplified ammo supply by only requiring powder hoists.
But, nearly always the shells stored this was were for a kind of ammo called "Semi-Armor Piercing" which was developed after the end of WW1 and was a base-fused shell that was not armed until it was fired. Hence it was relatively safe to store this way. A picture of the round can be found at
File:4.7 inch SAP Mk II A shell diagrams 1933.jpg - Wikipedia, the free encyclopedia
I have seen a number of pictures with the brackets filled with shells, but I can't put my fingers on any right now. Even in wartime, if in port, they were always empty so you can safely leave them that way on a display model. (Which really looks terrific by the way!)