In Vindicators, you are a tank and you go round blowing up other tanks and objects whilst collecting as many stars as you can. With the stars, you can customize your tank.
There's around 15 levels and 3 of them are "contest levels" where you can find the flashing "contest star". With that & 9 normal stars, you can play the contest.
I haven't got through the actual contest bit yet so I guess I dont deserve a t-shirt anyway.
The contest ended in Oct or Nov in 1988, according to the demo mode.