I would've been two years old.
Maybe Mum simply brought the Master System II at the time despite Mega Drive being new, because it was a cheaper alternative.
We were not poor, but not strapped for cash neither.
Back then no stores sold Master System here.
The ones who had it definitely some rich kids who bought it overseas.
Usually from Australia.

SEGA Mega Drive is the first SEGA's console to ever hit Indonesia's stores and everybody mentioned the console as "SEGA".
So back then whenever someone said SEGA usually they refered to the Mega Drive, not the company itself.
SEGA was synanomious with Mega Drive, just like Nintendo with NES here.
Of course that was three decades ago, things have been changed now.