I have seen guns from the 1790's with wood boxes, and even post-1800. The Lehigh/eastern Berks county area in particular will show post-1800 wood box lids, but if you see one from somewhere else, it should not be considered an anomaly. There are even "Southern mountain rifles" with them.
It was a matter of economy. Wood cost nothing. Brass was expensive. Also, a conservative buyer might prefer the "old fashioned" wood box lid, so, that's what the smith made him.
But yes, in general, wood boxes would be Revolutionary War period and earlier.