I’m bad at forum searches, but I’ve had some good luck with repairing Colt boxes. Elmer’s glue. Wipe off the extra. A little bit will work. Large diameter Rubber bands can hold tension for those split corner repairs.
This is what I was going to recommend. Similar to mixing wood glue with saw dust for wood repairs.I remember some of the stuff from that post. It talked about using the thicker carboard that backs pads of paper. Tearing it up into small pieces and soaking it overnight. Adding elmers glue to it so you end up with this thick goopy substance of liquid paper (but contains glue). You apply that goop to spots where needed and when it dries have you fixed those areas by filling in with actual paper.
WOW, you did a really nice job on that box! Some great info in this thread.Elmers or white glue. Colt's box makers did not put a lot into those old box's. Metal L braces on the corners to hold them together with large paper clips or cloths ping. Small brush works well, glue both sides of splits. Slow and patience. Before and after pictures of one of mine.