I've seen 3 varieties of this lens.
The oldest of them is the RMC.
Then came the SD
Finally came AF and improved optics in the AT-X SD.
Here's some general observations:
The old RMC may be limited to the K/M mount.
The SD can have "A", or not, but also has a Ricoh pin.
Fortunately Tokina had the foresight to make it a "bump" so that
it won't interfere with Pentax' AF coupling.
The AT-X SD is where auto-focus comes in. I've seen no manual
focus AT-X SD in the 400/5.6. (Someone correct me if that
observation is in error.)
The SD and AT-X SD are Very Good optically.
The RMC is much cheaper and OK optically.
Not bad, like old Soligor.
But imo it's worth the extra few bucks to get the SD.
Collin
KC8TKA
