The intended purpose of the T40 in the center of the drain plug is neither as a drain nor fill port, although I suppose you could use as an alternate way to fill the sump. Rather, it's known as a leveling plug. It caps off an internal stand tube used to gauge proper (warm) fluid level, I assume while the engine is idling. Basically you add fluid until it starts to drip out of the inner (stand tube) port, then you screw the T40 into the outer drain plug.

If you have time to kill, check out this video:

https://www.youtube.com/watch?v=Ehgwf_C5zgY

At about the 4 minute mark he starts describing the drain and leveling plugs' functions. It's a pretty interesting video if you like in-depth tranny talk.
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Current rides: 2005 Volvo S80 2.5T, 2003 Volvo V70 2.4NA, 1973 Volvo 1800ES (getting ever closer to road worthiness)