In what paper was OCB2 introduced?
You are correct: OCB2 was only named so after the fact. Confusingly, it was named OCB1 in the original 2004 paper.
You can find the OCB2 naming on the paper that introduces OCB3, The Software Performance of Authenticated-Encryption Modes:

Integrated AE schemes were invented to improve performance of composed
ones, but it has not been clear if they do. In the only comparative
study to date [31], McGrew and Viega found that their composed scheme,
GCM, was about as fast as, and sometimes faster than, the integrated
scheme OCB [35] (hereinafter OCB1, to distinguish it from a subsequent
variant we’ll call OCB2 [34]).

Another instance of the name transitioning is in two papers of Minematsu. The first one, Improved Security Analysis of XEX and LRW Modes at SAC 2006, refers to the mode as the original OCB1; a later paper written in 2008, Generalization and Extension of XEX* Mode, already refers to it as OCB 2.0.
Finally, a patent filed by Rogaway in 2005 refers to the mode as OCB 2.0, which is presumably where the name change happened first.