The marks are almost certainly for London 1781/82. The makers marks are obviously of a partnership but I'm unable to tell the exact letters except for the top left which is a T . It could be TL over IS which would make it Thomas Lawrence Jnr & James Sage's mark registered as smallworkers in March 1771.
The horizontal layout of the marks , with the makers mark at right angles rather worries me. I'm not familiar with this sort of item , but it seems more likely to be the mark on an item of flatware like a spoon, rather than a plate item. But I'll let others, more expert to comment