DRGM & DRP
The acronym D.R.G.M. with or without punctuation stands for Deutsches Reichsgebrauchsmuster, meaning that the design or function of an item was officially registered inside all of the Germany states and not only locally registered as it was the case before the introduction of centralized registration. Note that many people quote this acronym as standing for Deutsches Reich Gebrauchsmuster, which is grammatically wrong and also ommits the letter 's' after Reich. This results in shifting the weight of pronounciation on 'Deutsches Reich' alone, but this acronym has nothing to do with the Third Reich as many sellers want to imply so to catch the attention of certain 'collectors'.
D.R.G.M. registration was introduced 1891 and if you are dating items you should hold in mind that even during Allied occupation up until 1949, registration procedures remained untouched and still used the D.R.G.M. registration documents, which of course explains why D.R.G.M. marks can be found on products actually manufactured up until 1952 as the registration itself was valid for three years. As from the end of October 1952, all registrations were definately marked with 'Deutsches Bundesgebrauchsmuster' (D.B.G.M.) or simply with 'Gebrauchsmuster' or 'Gebrauchsmusterschutz', see below.
As already noted, the D.R.G.M. registration offered a basic copyright protection for the duration of three years and included the right to indicate the item status by marking the registered items with the D.R.G.M. acronym. It was left to the registration owner to include the registration number as the D.R.G.M. marking alone was the element with legal character. The actual result of such a registration (the form of protection) was called Gebrauchsmusterschutz.
D.R.G.M. registered products were protected either for their way of intended use or design only and this did not include patent protection. Patent rights were secured by applying for a Deutsches Reichspatent (D.R.P.), so even if many people use the term 'D.R.G.M-Patent' it is factually wrong. Reason for this mix-up was that the D.R.G.M. registration in colloquial language was also known as 'kleines Reichspatent' which literally stands for 'small Imperial patent' but actually was meant as 'poor people's patent' and made fun of the fact that many manufacturers could not afford the fees needed to register a full patent. One should take into count that German patent registration fees (as was openly criticized during the year 1906) where two and a half times higher than in England - and 36 (!!!) times higher than in the US.