Right now, "Previous build" tasks include all those tasks for which there are stamp files. However, there are 2 types of stamp files. In RP's words: "normal" ones and "sstate" ones. RP has suggested we should split them, and create a new category of Toaster tasks for the tasks with sstate stamps. In his words: "An alternative would be to log an extra "existing sstate" state and then mark this up a previously built task which came from the sstate cache. This would actually be my preferred outcome but complicates the UI and the code changes."
from RP's comment in bug 7355 https://bugzilla.yoctoproject.org/show_bug.cgi?id=7355#c16 the new category for tasks with sstate stamps should probably include "find tasks in previous builds" functionality, as the 'previous build' tasks do. I assume the list of tasks returned should only include cached tasks (and by extension, the list of tasks returned in the case of "previous build" tasks should never include cached tasks).