A final note, in reference to Rich's comment. As the colleague who brought this question to my attention pointed out, the tablet was excavated and the writing (for it is certainly writing) was inscribed while the clay was still wet (as Harry Falk clearly pointed out), so there is no question here of a fake: it is rather some writing in an as yet undeciphered script, just not, alas, Brāhmī (I say 'alas' since this would then be a [more or less] datable instance of Brāhmī not only from outside India but pre-Aśokan -- this was all too much to hope for!)
best thanks for the advice to all, Jonathan