Discreet 3dsmax 5
(code named Luna) was the first release ever to support the plug-in format of the previous version. Plug-ins developed for 3dsmax 4 could be used in 5 without a recompile, while both 2 and 3 required completely new versions. The biggest addition to 3dsmax 5 was the Advanced Lighting sub-system of the Scanline Renderer where two new plug-ins were introduced - a brute-force Global Illumination module called Light Tracer and a Radiosity module based on further research by the developers of Lightscape. (Historical note: Lightscape was acquired by Discreet Logic a couple of years before the Autodesk acquisition.) This also incl. Photometric and Day lights support. Further additions were
the inclusion of Reactor (previously a separate plug-in published by Discreet based on the HAVOK dynamics engine);
Set Key animation mode;
a refactored Track View with Curve Editor and Dope Sheet modes and
an enhanced UVW Unwrap editor
Render To Texture feature
New Named Selection Sets editor
New Transform gizmos
Character Assembly and Bone Tools
Spline IK
Gimbal rotation mode
Auto-Tangent interpolation
Improved Skin Modifier with Weight Table
Improved HSDS modifier UI
Support for Layers (taken from 3ds VIZ)
Ink'n'Paint Material
Translucent shader
On the human resources side, it is interesting to note that the product was developed under Chris Ford, previously senior Maya product manager who moved to Discreet when Alias dropped Wavefront. (He is now PRman business director at Pixar). Other related 3D Trivia: Bob Bennett, previously product manager for 3d Studio DOS, was Maya Development Manager for many years until the Alias acquisition by Autodesk - he is now with Luxology.
There were three point updates - 5.1, 5.1SP1 and 5.5 (the latter was the extended version with the Particle Flow extensions).
On 1 of the sites I found this description of the max versions. I became very interested in version 5.5 was it real or was it some kind of service pack for version 5? I saw information on the Internet only about 5.0 or 5.1 and then 6 3ds max and so on until 2022, which is now relevant.