The even numbers (68000, 68020, 68060) were reserved for major revisions to the 680x0 core. The odd numbers (68010, 68030, 68050) were minor upgrades from the previous chip. For example, the Motorola 68010 was a Motorola 68000 with some minor enhancements and modifications to some user/superuser instruction assignments. The Motorola 68030 was a Motorola 68020 with an MMU and more minor enhancements. The 68050 would have been a 68040 with some bugs fixed, which didn't really warrant a new name so it was sold as a 68040.