If you're on a 737 flying up front internationally, it has always been a toss-up. For example, one of the routes I frequent to central america, IAH-GUA, has been booked to F as long as I can remember (or cared). In other cases, like the old HKG-SGN route, it was booked into J on the 737. The amenities were always the cruddy domestic first amenities (with a some regional differences). But, you always have access to the UC if you are flying up front internationally (J or F). Note that the OP asked about the united club, not the polaris lounge. Not that it would have mattered -- "first" or "business" doesn't have "polaris" in the name.