L = left hand drive (LHD cars had the "S4" prefix replaced by either LBS4 or LS4).
S4 = Series 4
2906 = Chassis number
GT = This one I'm just guessing (as I've never seen GT on a S4 chassis plate before) means it had the 1600cc. Ford 2255E overhead valve engine with crossflow head and Weber 32DFM twin choke progressive down-draught carburettor in standard tune (GT Cortina) :shrug:, but IDK.