The Conway Stewart 560 replaced the 550 in 1963. As I have both versions, in the same colour, I thought a quick comparison of the two might be useful. In the photographs the 550 is at the top.
The 560 is noticeably longer, about 5mm., than the 550. The cap is also more streamlined. It may appear that the difference in length is due to the cap length but, as the next photo shows, this is not the case.
The barrel and section are both slightly longer on the 560.
As the 560 nib is 'semi hooded' and the 550 has a fully exposed nib, and they have a slightly different shape a size comparison is not clear but the impression is that the 550 has a slightly larger nib.
The size difference between the two pens is small but is significant enough to be noticed if the pens are handled 'one after the other'. In practice I don't think it makes a great deal of difference. Both pens would be more comfortsble when posted for use with average sized hands.
The shape of the sections is very different, it might appear that the flared end of the 550 may provide a better grip but, in practice, the fingers are normally in contact with the threaded area of the barrel so the section shape hardly ever comes into play.
Nevertheless, Conway Stewart must have had a reason for lengthening the Dinkie, perhaps it was in response to customer preferences or, possibly, an attempt to make the pen more modern by adopting a more streamlined design.
After only about a year in production the Dinkie 560 was abandoned in favour of the curious 'Lumina' model. Some may be attracted to the bright colours but many find these pens garish and not 'in tune' with the Conway Stewart image. the 'Holder Foreign' stamp that these pens carry is particularly off putting.
This 560 is on sale here and a red marbled 550 here