LSJ s.v. σύ notes 'in late Gr. σέν, Anatolian Studies p.76', which is SEG VI 370 (line 23), a fourth-century AD epitaph.
This inscription was discussed by W.M. Calder in 'The epigraphy of the Anatolian heresies' (pp. 59-91, with the delightfully quaint Pl. VII) in Anatolian Studies presented to Sir W. M. Ramsay, Manchester 1923, edd. W.H. Buckler and W.M. Calder.
See also GVI 1556 and MAMA VII 556 and X 77
Also of interest is the discord of a participle in the genitive with a noun in the dative (lines 4-5).