As Lancaster wrote, Gustav Genschow & Co, TM "Geco", was not a gunmaker, but a wholesaler and exporter. They had most of the guns they sold under their geco trade label made for them by the Zella-Mehlis guntrade, some shotguns they imported from Liege, so dating your rifle should be easy: Since 1911 the Zella-Mehlis proofhouse regularly stamped the date of proof besides the BUGN proofmarks. When you have the rifle in hand, take it apart. You will find the proofmarks and additional information under the rear end of the barrel. If the proofmarks are complete and legible, you will find sets of numbers: First, the bore (not groove or bullet) diameter of the barrel at time of proof, in case of a 9.3x57 probably a "9,0mm", followed by the case length "57". Next should be the month and year of proof, given for example as "2.28" standing for February 1928. Last there will be a ledger number for that month, running between one to four digits. If you find other numbers or letters, post a photo. some marks there may even point to the real maker.