As said, by 1918 almost every maker of the Gew.98 had been redirected to other projects and or severely curtailed production of the Gew.98. Even those that continued with G98 production, their production was far diminished over prior years. Mauser went from 270,000 rifles in 1917 to about 160,000 and they were the strongest producer. Amberg was the other "major" producer, easily the easiest "Imperial" maker to find today, they went from 240,000 to 80,000 the same years. The remaining producers are far more dramatic in their declines, some like Spandau essentially stopped in 1917. Most however produced trifling amounts in 1918, but all are very rare in "Imperial" condition. You can find these elusive 1918's with diligence, but in most cases you will have to settle for republican or NS era reworks, which typically means blued receivers anyway and they can't help narrow down this question.
Wolfgang has asked that we include this characteristic (blued "Imperial" receiver) in the Amberg study, we will of course, but I haven't set aside the time to go through my database for them and new ones haven't showed up, as i recall they are not common even among Amberg production. And most of these that could form a foundation to answer the question will be Amberg produced. This is not a simple project either, most 1918's were reworked after the war, which means they were blued, Amberg is a little different, thanks to Bavaria being rather obstinate, they refused to give up on the Einwohnerwehr (Bayern) and the conservatives and the few proto-nazis (nationalistic socialists) like Röhm, who helped arm the EWB and later became an arms smuggler, hid large stores of small arms, many of these seemingly were not discovered to be "reworked" by the government. Further the communists were also able arms hoarders, as were the conservative elements throughout Germany... it is not clear how many of these rifles escaped WWII, though there are quite a few 1917-1918 Amberg's in Imperial trim that were EWB branded, I have owned a couple over the years and they show up fairly often.
To complicate this further, most of 1918 production never left the depots (which means they figured prominently in 1919-1924 destruction totals as they were in government hands and we know millions of rifles were destroyed, many before the IAMCC was created...), very elusive to find a 1918 dated G98 in Imperial trim, I doubt there are a dozen recorded in my database outside of Amberg/Mauser. Danzig alone went from nearly 400,000 to less than 60,000 during 1918! The rest are little different in scale, Suhl makers dropped like a rock, the consortium makers can be found, but like Oberspree they are some of the rarest maker dates known, Oberspree made the same numbers as the SC, about 40,000, but seem more difficult, you see "blackened" receivers with their production also.