In the genitive, presumably quoted from somewhere.
Latin magister appears in Greek in various forms: most often μάγιστρος (alpha 2107 et al.), μαγίστερ (mu 16), μαγιέστερ (mu 8), μαγίστωρ , and (as here) μαΐστωρ . The loss of gamma before iota could reflect a sound change either in late Latin (cf. Italian maestro, French maître) or in Greek.