The version released in 2000 is basically a service fix of the Navision Financials 2.50 release. In it's "native" database mode, it was a very stable version, still in use by some customers.
Besides the "plain" version 2.60, the the following service releases was also made: 2.60A, 2.60B, 2.60C, 2.60D, 2.60E (do you have more details about them, please comment).
The new functionality of 2.60 is in the new Navision Manufacturing and Navision Advanced Distribution sub-versions.
Navision had a reputation of delivering simple reliable fast software. Navision 2.60 AD broke all these traditions. The design was poor, and even if you did not activate the AD functionality, the system was extremely slow and very buggy. The AD modifications did not follow Navision Guidelines, and data corruptions became very common. Core to most of the problems, was extensive modification of Tables 36 and 37 to do things they were never designed for, like transfers. General consensus would put this as the most buggy and by far worst version of Navision ever released.
A core issue of 2.60AD is that the Advanced Distribution enhancements very rarely fit to a companies needs, but making even the slightest code change generally had an on going domino effect such that the NSC would be continually fixing bugs.
Like Manufacturing then AD was basically a "stand-alone" version. That also made it nearly impossible to combine manufacturing AND advanced distribution. An issue that wasn't fixed until Navision Attain again combined them into NAV.
The Navision Manufacturing module had a limited US release with version 2.50.
Version 2.60M was 2.60 with the Manufacturing Add-On integrated. It basically worked, but was generally too slow to work for the companies it was designed for, so was not too successful. It did though give a good test base for the future version to come.
2.65 continues to improve the SQL Server options made available with Navision Financials 2.50 in 1999. The SIFT-Technology used by Navision did not exist in SQL Server at that time, and caused flowfield calculations to be very slow.