Not Hannover; we do not know the exact purpose of these extra stamps, however the late great Joe Steen surmised that these were some form of pre-inspection or re-inspection, a qualifier of sorts (qualifier was the word he used). Danzig made enormous numbers of G98's during the war, easily the top producer 1916-1917, especially in 1917. They also rival Erfurt in crudeness and issues with fit, one of the few mfg that markings like BC's can be seen above the woodline, - generally a bunch of cosmetic crudeness, but a sign of how these large numbers came at a cost. Erfurt and Danzig were the two largest wartime makers, both have a reputation for lower standards, these extra markings may be connected to this tempo and its costs.
In short they are believed to be counter inspectors, crown over letter is an inspector who supervised teams, these extra stamps could be a sub-inspector for that task. It is well known and acknowledged that inspection was conducted on samples, or lots, not every piece exactingly scrutinized, - if a problem is discovered the decision to expand the lot would be considered. This was common practice in all militaries involved in wartime mass production. I once read a detailed article in Army Ordnance that covers this practice in US Army ordnance inspection, unfortunately dealing with German armaments production little of this has been scrutinized in detail, though Storz does touch on it in his Modell98 book, an interesting passage or couple paragraphs on this subject, - even if his translation is like piecing together a jigsaw puzzle...
Regarding "Hannover" and the other situations where a letter denotes the assembler, - this is always a letter or couple of letters in some cases, at the top of the receiver. In the same general position a star is found on a sterngewehr. Numerous examples exist, E=Erfurt is common, some pre-war 98a are known, several G98's also, but they are rare. Typically, extra markings on the RR denote depot work or if an extra crowned inspection, a re-barrel is typical. Markings under are uncommon, but usually any extra markings are sign of depot work, often interwar depot work.
There are variables in all of this, most of which we barely have a grasp of the basics...
