First of all any 68 Shelby can have Lucas lamps (period correct lenses, housings.wiring) and be acceptable for concours judging. If I had to make a choice on a 4/3/68 car I would use the Lucas but it could have had the Marchals originally. From my data charts the latest Marchal equipped car I have recorded is 3/28 so we are only talking a week plus the dates we are using are Ford production dates not AO Smith where the lamps were installed.