Probably a combination of factors: parts were changed to whatever was available, while making some improvements in production and keeping costs down and maximizing profit margins.
Within the first year of production, Marantz (or CEC rather) put the 6300 through several changes:
The component configurations/locations changed slightly on the photo unit (H002) and power supply (P-800). The tonearm assembly changed and the parts are NOT compatible from early production to later models. The gimbal assembly boreholes and tonearm wand lengths are completely different from one another. Also, the feet were changed, the 45 spindle adaptors went through several revisions and of course, all the stuff you already mentioned.
The control panel grommets are also another thing to note. If it's an early production model, the grommets for the push buttons will be glued underneath the control panel, hidden from view. If it's a later production model, the grommets are visible from the top like yours.
The motors were also changed twice that I know of...early 6300 production models used a discrete design that were also used in Pioneer PL-71's and JVC JL-B44's. The later 6300's were changed to an IC type using the AN620 chip - which is an old Matsushita IC. I have yet to find anyone who has any knowledge of a replacement IC available for the AN620. I happen to have both 6300 versions including their motors, but am currently using the early model.
