Apparently, there is an academic point as to whether that one was an actual conclave, or whether the first conclave took place immediately after it, new rules having been put in place, or whether they properly began in 1404 and so on 🤔
"The election occurred more than a year after the magistrates of Viterbo locked the cardinals in, reduced their rations to bread and water, and removed the roof of the Palazzo dei Papi di Viterbo where the election took place.
As a result of the length of the election, during which three of the twenty cardinal-electors died and one resigned, Gregory X promulgated the papal bull Ubi periculum on 7 July 1274, during the Second Council of Lyon, establishing the papal conclave, whose rules were based on the tactics employed against the cardinals in Viterbo. The first election held under those rules is sometimes viewed as the first conclave."
https://en.wikipedia.org/wiki/1268%E2%80%931271_papal_election