YouTube has several videos on dial caliper repair. https://www.youtube.com/watch?v=vYpe0uNpXyQ is one but I don't believe that your bezel is held on by an O ring.
This document shows exploded views of different vintages of your model: https://www.mitutoyo.co.jp/eng/support/service/parts_list/result.php?code=505-626&shorui=505-21 I can't be sure but it appears that the bezel is held on by a screw at the top of the dial ( part #'s 21 and 22 in the first document). Good luck.
WOB
Have a look at the Long Island Indicator website, particularly this page here http://longislandindicator.com/p233.html on replacing indicator crystals. It goes through the many different ways bezels may be held on and removed etc. Might give you some ideas to try out.
Otherwise, might be a case of "Insert credit card, buy new caliper".
So, how did you get the bezel off in the end? (Just in case I ever need to.)
Apply pressure from bottom of the bezel at 2 points 180 degrees apart. Press hard enough and the bezel is released.
Sorry for the wrong advice. I am glad you have it open now, next time it will be easy! What are you going to do with it now? Clean it, or is there something not working.
Wow that is a small pinion, especially if you know how small an Aussie 5c coin is!
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