Doesn't match the registered mark of any LBMA-approved list refiner and/or assayer, at least the current ones. Do you have any information on the provenance of this item? Does the underside have a stamp of two crossed pickaxes over an oval? I have a hunch the 'L' is Laughlin, and the bar was sourced from Nevada.
Another track of investigation, possible it is an old Logam Mulia. http://www.logammulia.com Though they are LBMA-approved and as The Roswell Crash Survivor indicates, it's not their (current) registered mark.