D5 (D5244T to 2005) Auto transmission fluid level check

[Newbie Tony]

Hi guys.
I have a 2003 XC90 D5 AWD with 180k miles on the clock and I decided to check the fluid level in the transmission today. Having consulted the Haynes manual I removed the air filter housing looking for a T55 filler plug but instead found what appears to be a deliberately unusual bolt head not designed for conventional sockets.......designed to prevent DIYer's from undoing? Underneath I didn't find the T40 level check plug housed within another T55 drain bolt as Haynes said I would, nor anything resembling a drain plug. After a bit more searching I found a dipstick at the front of the engine and gearbox saying "ATF Fluid"
I'm trying to enclose a few photos below showing the dipstick level, the unusual bolt and something else that I know nothing about.......potentially an overflow?
I've got a syphon and have taken about 150ml out as the dipstick shows it over filled after a 15 minute mixed run. Despite removing some oil the level on the dipstick doesn't seem to have dropped (I'm wiping clean then reinserting on level ground). I'm going to wait til it's cooled then recheck the level. The fluid I've taken out looks quite black but doesn't smell burnt to my inexperienced nose!
My questions are: Am I doing the right thing? Is there a drain plug somewhere? I've not done any work on auto transmissions before so thanks in advance for your help!
Cheers Tony