Is that a center tap tranformer just labelled differently?
Not necessarily. If the transformer is rated to produce full output from either the 12V tap or the 24V tap, it could be that the winding from the 12 to 24V terminals is of a finer gage, since the current that could be drawn from the 24V tap would be half that you could draw at 12V. This you can determine by comparing the resistance of the 0-12 and 12-24 windings.

Would that matter for using it as a center-tapped +/- 12V transformer? Not much, the terminal voltage at the high side would be fractionally lower under load, but the overall heating compared to drawing the full power at a single tap would actually be less.