MATTA 26: Boeing RC-135W with tail 62-4138 working with ATCC Dutch Mil on 128.3500 MHz. At least for me this is a callsign I never heard/logged before.
That is because it is a one time callsign only. RC135's are flying almost daily out of Mildenhall since late April to Baltics. every day different callsign.
Callsigns used since April 26th:
EXTRO MARCO SEPAL HOSE HONE NAVEL ORKA SNOWY UNCUT GAUL HOST GULP CARA EMMER LEND LORUM BONDI TONED JOTA GAMUT TALUS CURD ALONG AWFUL CARTY NOVAR LOOP FINCH NUMB DIRE JIMA PAGAN STRUM SEND HARP GATE GAUDY HOOKS PIZZA VELAR PONT BUILD BARF VOLVO CUTCH ABASK DEMIT STOLE ATNA DIVA DRYAD MATTA