Hi Nigel,
Basically there's not much more to report apart from the fact that the 20 inch machine by the time when yours (Series II) was built was pretty much in demise as the cost was high and the mower market had nearly completely moved to rotaries thus the preferred size was the 17 inch unit for those that still considered the reel mower the best type available. Rest assured though the 20 is the best if you have the size lawn that will work well with the large size machine.
Cheers,
BB.