Just dug this out of my archive of use full info
Canon 300mm f2.8 L (NON IS)
Optically and mechanically there are no differences between the lenses and Canon only considers there to be one "full" model of the 300/2.8L non-IS.
However they did release three "sub-models" for lack of a better description. These were marked on the external box and also on the warranty card.
The original one had no numeral after its name, but the second and third ones were marked as II and III respectively.
Differences
MK I
The original lens has a metal hood and no strap slot opening on the back of the tripod foot.
The case for these lenses was a black vinyl-coated hard case.
This lens also has Canon's brown coating on the front element.
MK II
The MK II changed to a polycarbonate hood (lighter) and added the strap slot in the tripod foot.
These lenses came with a green aluminum-trimmed lens trunk rather than the hard case.
The front-element coating changed from the brown one to Canon's green coating during the MK II production run.
MK III
The III was very similar to the II but came with the newer beige trunk instead of the green one.
Hope this is of intrest