I can't think of a current MF system that has anything above a 300 (except for a couple of exotics for Hasselblad V-mount that Zeiss built a few of to prove they could do it - including a one-off 1700 mm f4).
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There are 3 Rollei 6000/Hy6 lenses above 300 that I know of:
The 5.6/350 Tele-Tessar,
the 8/500 Tele-Apotessar,
and the 8/1000 Tele-Tessar.
They occasionally turn up in the second hand market.