Maps from 1804 and 1885. The road that runs by Maen Llia (Llech Llai on the older map) is absent on the 1804 map. It is almost certainly a toll road built around 1840 or so. There must have been a track there before then to connect the now abandoned Llechllia farm, which has no inroads on the 1804 map.
I suspect this track was originally ancient for a few reasons, but mainly because there are several significant stones on its wide verges.
Image credit: cerrig