this is the lid of an airtight container for shellcases, german, WW2.
In my opinion, for 15 cm Tbts. K. C/36, Tbts K/L 45 and 15 cm S.K. C/28.
The "Lg"tells us that this was once the lid for a container containign a shellcases for "Leuchtgeschoss", a flare shell.
The 'Lg"powderloads were lighter and used a different powder in comparison to those used for normal shell.