Traditional Glue pre 1940 is Rabbit skin or hide glue. Today Children's Craft PVA glue or Wallpaper Adhesive, They all have one thing in common they are reversible with water or steam if the case is used for another gun needing a label change. One adhesive I would not recommend is standard PVA this is problematical to remove cleanly. There may be other types of reversible glues but I do stick to the ones I know that truly are.
I wonder if that is the same thing as what used to be known here in the States as mucilage glue. It used to come in a glass bottle with a red rubber slotted spreader tip, for applying.
