This is a common GM/Opel/Saab unit also, One of my Saabs has this one in it and same deal with the plug that looks like a fill plug.
I hate filling that thing through the small dipstick. Clinebarger hooked me up with a flexible dipstick tube with dipstick that was easy to rework to fit. I mounted it on the left side of the radiator, it made life so much easier.