I'd assume it's at the board edge, as, even if the fins were flush against the board (not a show stopper, but requires forced air and ducting, PITA) the pins won't be long enough to still reach the board for any reasonable height of heatsink.
Much more common to mount the device underneath (leads bent up), so the PCB can be mounted to the heatsink with standoffs. Or at the board edge, PCB can be flush mounted to the heatsink.
Example: well, not a great one, but you can just see it anyways.
Note the large holes in the middle of the board, through which tabs can just be seen.
To minimize lead strain, the devices need to be soldered in place after fixing to the heatsink.
Tim