The Germans were coerced (and deceived) into signing Versailles, it violated the terms of the armistice and the entire premise of US involvement in the war. Very few things united the German people after the war, one was an eternal hatred for France and Poland, another was (perhaps most of all) Versailles and its implementation (which came with shocking violations of German sovereignty, - not just superficially like what happened to France after Napoleon or 1870, but humiliating so, regulations & laws that boggle the mind were forced upon the new Republic... all of which weakened it to adversaries like Hitler and the communists - Hitler's often stated that his primary objective was the destruction of the Republic, not confronting France & Poland, he showed this consistently throughout his rise to power)
Germany was fragmented into dozens of splinter groups, almost all of them hid small arms, especially those on the right and the communists. At Spa (July 1920) Germany admitted that not only did Germany possess hundreds of thousands more rifles than allowed, but more than a million were unaccounted for (see my post on the property marking, Seeckt boldly confronted the Entente leaders on the subject of weapons - the only one of the German delegation not kowtowed by Lloyd George and his French criminal collaborators - the chaos created by the Entente's actions made it impossible to comply with the terms dictated upon Germany), The entire (and only) purpose of the 1920 marking was to prevent theft of rifles for the rewards offered for civilians to turn them in (police or soldiers selling or having rifles stolen for the rewards). Nothing else, there was no registry, no allied supervision of German inventories (not in a systematic way where Allied soldiers kept inventory or counted/check unit inventories - though they did perform spot inspections, looking for hidden inventories), - such ridiculous schemes advanced by ignoramuses regarding the marking illustrate the level "research" into the subject. The IAMCC consisted of a few hundred men usually (numbers fluctuated but never more than a couple thousand), the US took no part in it and eventually rejected anything to do with the vengeance being perpetrated upon the German people, Hoover especially was vocal regarding French activities... most of the IAMCC work was done by German counterparts whose work was supervised (confirmed) by inspectors. Rifles were a trivial issue and most of the concern was the insanity going on in Germany, the riots, the fighting, the instability, much of which overflowed into the Rhineland occupation zones. Worse of all it was hampering the Republics ability to service the reparation payments, - most of which went to France and Belgium.
Regarding why some rifles lack this property mark, the reasons are endless, many rifles (MG's artillery and everything else imaginable) were hidden by political factions, some state owned (like Bavaria who steadfastly refused to comply with orders from the central government... The far right EWB refused to disband, and when it did they "lost" a lot of rifles... Prussia was little better but on the opposite political spectrum), Many political factions on the right had close ties to the Reichswehr, especially the RM (navy), eventually most of these rifles would find their way back into government hands, still illegal, but after Seeckt was pushed out the Reichsheer developed closer ties to the government under Stresemann and most of the details were known, even the English knew and didn't object too strenuously (most of the rifles and equipment was in the east facing Poland, Frances assbuddy, and England wanted German recovery more than German subjection), besides newspapers had reported on many of the armaments scandals, and the French and Poles were adept at spying in Germany (Germany had been Frances plaything for centuries, meddling in Germany's affairs ruthlessly, plus Germany always had problems with divisions, catholics vs. protestants, which France took full advantage of... the widespread distrust of Prussia, - Bavarians, Württ, Saxons and Rhinelanders alike didn't approve of Prussian dominance).
There is also the police, the most scrutinized institution in Germany was the police, naturally this scrutiny inspired further secrecy, - France especially took an interest in police activities. They rightly considered the police as a ready reserve for the Reichsheer (so did the Germans - it was one of Hitler's first moves to take over the police), constant bickering between the Germans and the Entente ensued over the question and arming of police, the Germans would stall, when pressure built enough (which usually meant when the English took sufficient interest) they would reorganize or disband and reinstitute, no telling how many illegal arms the police held. Even after all the pressure was gone, the IAMCC left, Germany in the League, etc... the police would be a source of irritation between Germany and France.
There is also the likelihood many rifles were never marked at all, Germany was grossly in excess of allowed limits, most that were marked were destroyed eventually. Even in the Rhineland/occupation zones, or demilitarized buffer police existed, it is doubtful these rifles were marked. The police under Entente supervision were heavily scrutinized and restricted, there were dozens of police-like forces throughout Germany, -forest, river, border, railroad, rural, city and federal, who knows what else... when you have a country where river piracy and smuggling is a serious problem you are going to have a lot of police organizations.
