That's correct about a few of the route 55 services extending to/from Acland St. However, in the 60s and 70s, it was limited to a few peak services, with the remaining West Coburg services terminating at Domain Rd (as it was then known). Before the interchange was built, they had to turn into St Kilda Rd and shunt just past Domain Rd, often causing a bank up of trams (which was made worse because they then had to wait for the lights to permit them to turn into Park St).
In the MMTB public timetable of September 1976, the am services on the route to William St left St Kilda Beach at 7.50, 8.05 and 8.22. The timetable doesn't state that all were through routed to West Coburg, but certainly at least some of them were. Interestingly, it refers to the route numbers applicable to this route being 53 and 55. I assume that route 53 terminated at Dudley Street (but I can't recall ever seeing this number in use).
There were even fewer services in the pm, leaving Dudley St at 5.04 and 5.16.