The only known instance where the rune does take the value of Latin x in epigraphy is the spelling of rex "king" on the interlace coin dies of king Beonna (mid 8th century).
But the sceat fabric survived in East Anglia under Beonna and until the mid 9th century in Northumbria, while the new-style coinages were not struck not only by Offa, but also by the kings of East Anglia, Kent, and Wessex, by two archbishops of Canterbury, and even in the name of Offa's queen, Cynethryth.
Beonna |