I understood that the Old Roman Catholics had just about terminated operations, thinking there was no further need for them ... but then Vatican II happened, and they retained the Tridentine Mass.
Yes, this is correct. Archbishop Bernard Mary Williams, the successor in Great Britain to Archbishop Arnold hαɾɾιs Mathew as leader of the Old Romans, came to believe by the 1940s that the Old Romans had no reason to exist any longer because circuмstances that had necessitated the existence of Old Romans had ceased, that ordinations would cease and all Old Roman congregations would be rolled back into the official Roman Catholic Church.
However, the ascendency of Liberalism and Modernism with Vatican II obliged the Old Romans to alter course and resume a full apostolate for Tradition.
The history of the Old Romans is a bit different in North America. After +Bernard Mary died in the 1950s, Old Romans divided into two different groups in Great Britain. However, the North American Old Romans trace themselves through Archbishop Carfora and Archbishop Landes Berghes, the latter having been sent to the USA early during the First World War by +Mathews because +Landes Berghes was an Austrian national and, thus, enemy national in Britain during the Great War. Most North American Old Romans receive their Holy Orders from +Mathew (who was consecrated by Archbishop Gul of Utrecht) through +Landes Berghes and +Carfora, whereas most UK and continental Old Romans receive their Holy Orders from +Mathew through Archbishop Shelley and Archbishop Paget King. Although, there is plenty of crossover in the ordination lines, which tend to be short and clear, unlike so many odd-ball "Orthodox Old Brazilian Catholic Apostolic United Chaldean Church of the National Celtic Ancient Eastern Templars" groups of dubious orthodoxy and even more dubious validity in Orders and sacraments.