Complete, new old stock, CEV 43415 tail light assembly.
Found on several Italian imports like the Garelli TEAM for example. Should fit some Derbi DS50 as well but compare to your original.
Base is badged CEV 42942. Lens is badged DGM 43415 LP with a CEV stamp that has 329 to the left, 328 above, and 330 to the right of it.
5" wide at the top. 5 3/4" wide at the bottom.
Mounting holes are 105mm apart from center to center.