The camera itself can be set up to ISO 6400 but the ISO limit for TTL flash would be that of the flash unit that is compatible with TTL operation. For the Pentax AF280T and AF400T flashes this would be ISO 800.
Thanks. Where did you get that info?
Pentax 645, Pentax 6X7MLU, and many Nikons-F2 Photomic F2AS FM2N N2000 N6000 N6006 Nikomat FTN