https://www.eecs.mit.edu/academics/subject-numbering/ says that they changed things from when I understood it, again. Looks like they completely renumbered every single class so they could create a simple validator because no number was reused between old and new.

6.0002 is now 6.100B (the B is deconflict) but most everything else went to four digits, with the last digit a 0 for undergrad and a 2 for graduate school taking the same class. e.g. 6.6830 is the undergraduate Computer Vision class and 6.6832 is the graduate Computer Vision class.