Raimey, the stamp sais only "Express" without "non raye"! The marking "Express" only sais it was proofed for one of the blackpowder Express cartridges, which all took a healthier bp charge than the proof tables prescribed for the "ordinary" proof. Remember the German use of the crown/E mark for higher-than-normal pressure proof. The 1891 German proof tables, patterned after the Liege ones, prescribed for gauge # 99.7 =.360"bore an "ordinary" final proof charge of 89.5gr BP/403gr lead bullet, but for the "Express" final proof 100.3gr BP. I think the barrel was marked "8.9"by the barrelmaker, but at the proofhouse for some reason only the 8.8 mm plug gauge entered, so the two designations.
According to Gargela/Faktor:"Zeichen auf Handfeuerwaffen", the EXPRESS NON RAYE mark apeeared in the 1894 tables, but was included no more in the 1924 tables. If it was still in use all the time, I don't know