Good clarification...
The Sarken is available in sidelock (moving back heel bail w/ plastic lever thingy) and spirlock (no plastic thingy)... embarrassingly enough I have both
The Dart is only available in spirlock...
so if you have a Sarken spirlock (or dart) already save yourself some cash and buy only the front piece and rotate the back piece...
Note: if you have neither and plan on purchasing both buy the complete sarken as it comes with the antibot plates attached where the dart doesn't (which costs more anyways).
If you have the sarken spirlock (which it sounds like you do Don) then you are stuck buying both...
PS I love my darts. And sarkens.