The Luger has served in both World Wars. 1918 is the date it was manufactured. 1920 is the year that the new German government took accountability for guns left over from the first world war and marked such guns with the 1920 date to identify them as Government property... Erfurt was a military arsenal (rather than a commercial gun manufacturer pressed into service for the war effort) and is the manufacturer of the gun. I hope this has been helpful.
Should you decide that you still want to sell the Luger... you can offer it here in the For Sale forum. Procedures for doing it legally can be explained should that be what you decide to do.
To disassemble it: FIRST make sure it is unloaded... the rest of the procedure is HERE:
http://www.lugerforum.com/disassembly-1.html
(Step 4 SHOULD read to get the GRIPs off... not the safety catch.)