On page BE33 of my factory 120Y manual there is an illustration of the instrument cluster & the letters BRK can be made out in this light. I needed a magnifying glass to read it, but it is clear, ... just small.
This is the top light, between the temp & fuel guages which are just to the left of the speedo.

This is the Australian manual & this light wasn't used, so it's purpose is not explained, but this particular illustration must be a 'generic' one for all markets.

Is it for brake fluid level warning, brake circuit fail, or hand brake? I can't answer that, but it's a brake warning light that obviously is used in some overseas markets.

Edit
I have just been checking the factory manual for 1200's & some cars had a "hand brake warning switch" as well as a "service brake line pressure differential warning switch" [brake circuit fail] These were wired through the same light on the 1200 coupe instrument cluster, so the unused warning light in our Coupes is for both the 'hand brake on', & 'service brake circuit fail' function.

I now speculate that the unused 120Y light was used for the same thing.

I hope to use this light in my coupe for the auto trans 'overdrive on' function. [Jatco 4N71B]