There isn't a direct synonym for "Stanley prune," as it is a specific type of plum. However, there are several other types of plums, such as blue plums, Japanese plums, and Damson plums, which could be used as substitutes in some recipes. Additionally, if "Stanley prune" is being used to describe a specific flavor or texture, alternative descriptions such as "tart" or "firm" could be used instead. It is important to note that substituting one type of fruit for another may alter the final result of the recipe and should be done with caution.