Lucas Headlight lens weird

Bought this today at Oley swap meet. Miller ring with Lucas lens but in script is "La Mans 24" and at the bottom is Made In England. It was cheap and I like weird things. Anybody know what it fits and is some sort of commemorative, or from a Moto Guzzi La Mans bike? the diameter is approx 7.5 inches Thanks

Have one in my '61 Matchless G3.