The transformer is a 'typical' example of primary over-winding.
Given the wide range of global AC mains standards - anywhere from 110V to 250V (possibly wider) - the manufacturers have additional taps added to the main winding which allows the user to select the correct (or nearest possible) primary input voltage.
You have a FIXED primary mains supply (quoted as 230VAC) therefore if you use a different primary voltage tap you get the equivalent increase or decrease in output voltages.
With 230V applied you show that you get 15VAC out.
Your diagram states 12V and 18V for the other inputs and these represent a 20% variation lower/higher which, when applied to the primary, mean the other taps are meant for 190V and 275V inputs respectively.
The BRN and R taps will be 'in between' taps and only have a nominal difference of 20V between them so CANNOT BE USED together to apply the mains input.
I suggest your transformer has a primary something like:
BLK -0V (neutral)
R - 210V
BN - 220V
GN - 230V
BU - 240V
Y - 250V
hi Kelly's eye
Your observation may be correct, regarding BLK to Y, it could be tapping for varied AC inputs, but BRN & R on the primary side are actually secondary ones. My measurements of resistance as below:-
1.(primary side) BRN-R - 1.2 ohms (it cannot be a primary)
2. (primary side) BLK-G - 8 ohms
3. (primary side) BLK-Y - 7.3 ohms
4. (primary side) BLK-BLU - 6.8 ohms
5. (secondary side) BRN-BLU - 0.8 ohms
6. (secondary side) BRN-RED - 0.8 ohms
I hope this will clear you doubts...