Probably a diverted or short 57 where the desto wasn't changed to 57a. B2s have been to the West Maribyrnong terminus before but they don't usually run the usual route through North Melbourne. Running a 57 via Haymarket is a possibility.
Doubt it was a diverted 57a, as I saw it about three kilometres past where the 57a turns around. The destination definitely said it was a 57 and going all the way to the West Mariibyrnong terminus.
It may well have gone via Haymarket though; are B2s still physically incapable of running the route via North Melbourne? Is it the curve at the beginning of Abbotsford Street that gets them?