They swapped in the 752s a few years ago for that time of year (
link to thread) but without making them full ps service and just sold it as F. IIRC there was another thread where they did 6 weeks with the 752s once a day in each direction (maybe 2016?). So unless UA says they're doing ps, it might be regular F in a 752, which I got a few weeks ago IAD-LAX for an upgrade price the same as the low end on a 737 (~$50/hr).