RB133 Isnt officialy supported in RouterOS v6.x as stated on Mikrotiks downloadpage:
"RB111, RB112, RB133C, RB150 devices are not supported in v6, please use v5."
RB133 and RB133C are different beasts.
Cosmetically different beasts then, differences only is in amount of Memory and numbers of MiniPCI slots as far as I remember.
But it is entirely possible I might have misunderstood, and either way that they OFFICIALY doesnt support them doesnt necesserily mean that it dosesnt work!
But I would anyway not have relied on either type of board on anything critical in a production envirnoment now anymore.
All 133/133c boards I ever had ended up getting slower and slower and slower for no apparant reason, till just logging in used all the CPU for minutes at a time. If I were lucky I got in, but if I was unlucky it disconnected and I had to start all over and hope for the best.
And for every upgrade of RouterOS I did it just got worse.
Needless to say all but a very few of my 133/133c boards have long been replaced with newer boards.
(However thinking back on it now it is actually possible that the normal 133 boards behaved a little better then the 133c, but in the end they all got the same treatment. Exchange!