They share the same characteristics as the Amberg receivers (under the wood line at the joint where the siderail starts). The purpose is not known, but they seem to have some purpose with assembly, some seem to have a dating characteristic (4C8- the 8 being 1918, - most are 1917 or 1918 dated and end with 7 or 8 respectively). Wolfgang was working on this and I was trending them but haven't recently.
I doubt the purpose would reveal much if it were pursued, as it probably served some purpose for the manufacturer, assembly perhaps, but I can't see it aiding in the understanding of the rifles today.