A "lamb" is a young sheep. Here is it used symbolically to refer to Christ. See how you translated this in [Revelation 5:6](../05/06.md). (See: [[rc://en/ta/man/jit/writing-symlanguage]])
This can be stated in active form. Alternate translation: "on whose foreheads the Lamb and his Father had written their names" (See: [[rc://en/ta/man/jit/figs-activepassive]])