A brief supplement to the Two Oldcastles of London.

This article focuses on the authorship and textual history of the early modern history play *Sir John Oldcastle* (SJO), reexamining previous stylometric analyses with the introduction of the General Imposters (GI) method. Historical records attribute the play's first part to Anthony Munday, Michael Drayton, Richard Hathwaye, and Robert Wilson, but stylometric evidence, including n-gram comparisons with Shakespeare's known works, suggests a dominant Shakespearean authorship influence, possibly in collaboration with William Rowley. The article discusses the complex publication and performance history, including the misidentification of *Sir John Oldcastle* with *Henry IV* due to political sensitivities surrounding the Oldcastle character. The GI method, a computational authorship verification tool, supports the hypothesis that the extant quarto (Q1) of *Sir John Oldcastle*—likely the second part—bears strong stylistic resemblance to Shakespeare, while earlier parts attributed to other playwrights may have been lost.