In another thread on my BT300, @Archer saw that the main plate in my 552 movement had a hole where there should not have been one (between the retaining screw and stem)--the plate appeared to be off of a 56x movement.

https://omegaforums.net/threads/big-triangle-165-024-arrived-insight-appreciated.41799/

After doing some digging, I found two examples of 300s with the hole in the main plate, which caused @kox to come up with some theories, leading to more questions from @Archer.

I poured through ebay completed auctions and found about 15 examples of 25,6xxx,xxx or later 55x movements with the hole in the main plate. It appears to be some sort of anomaly from the factory, but why?

For any members that have pics of their 55x movements that are post 25,xxx,xxx, it would be great to see if the "doughnut" hole is on the main plate.

Talk about getting into the weeds, but this is what makes this hobby so much fun...it not everyday that you uncover some idiosyncratic aspects of mass produced movements.