Hi Howard,
For no explicable reason, when the Spark was designed the transoms were raked at a different angle to the transom of the Dart, despite the apparent intention to use Dart rudder assemblies and blades on the Spark (I would have expected the rudder blades would work best at the angle for which they were first designed on the Dart!). In order that the tiller arms would be at a sensible angle over the Spark transoms and rear beam it was necessary to use a longer sleeve (see
http://www.windsport.co.uk/modules/item ... img_id=629) which slides over the tiller post on the upper rudder stock casting (the Dart part which will be on these Ebay advertised rudders will be as per
http://www.windsport.co.uk/modules/item ... img_id=933). It's my understanding that this is the only difference between the Sprint 15 and Dart 18 rudder assemblies, and you would need to replace the shorter sleeves on these advertised rudders to use them on a Sprint 15.
If you look at "Section 4" page (
http://www.sprint15.com/manuals/parts_list/section4.jpg) of the manuals on this website (it uses Lase Centre part numbers...) you can see the critical differences.
Hope this helps (I suspect that if it does, it may increase competition in the Ebay auction........)