Reproduction 8mm go/no go muzzle bore gauge in a resin 3D printed replica of the Bakelite holder. This is what is supposed to go in the little clips on the lid of your MG gunners pouch. This gauge would be used to test MG barrels to determine if a barrel was worn out or still usable. If the gauge doesn't at least start fitting into the bore, the barrel is possibly defective or not 8mm. The deeper the gauge goes into the bore the more worn out it is. If the gauge falls down the bore, then the barrel is completely worn out. Original examples are extremely rare.
The tubes did turn out with a sort of fish scale looking finish from the layer lines of the print but are otherwise spot on to original.