There's nothing on pin 3 of the DB25 connector.
The Miracle Piano pinout is as follows, for sake of comparison, along with official Nintendo controller wiring colours (which in actuality mean nothing, esp. if you're using an aftermarket controller connector -- you tell us!):
Code:
DB25 pin 1 = supposedly unnecessary
DB25 pin 7 = NES controller pin 1 (ground) (brown)
DB25 pin 10 = NES controller pin 4 (D1/data) (yellow)
DB25 pin 13 = NES controller pin 3 (OUT0/latch) (orange)
DB25 pin 19 = NES controller pin 2 (CUP/clock) (red)
We know definitively that the Miracle Piano uses DB25 pins 7, 10, 13, and 19. So, to me, it really looks like what you have is a Miracle Piano adapter, with one difference on yours (again assuming colours match official spec): DB25 pin 1 is connected to NES controller pin 7 (VCC/5V). That's the only difference.
If this was intended for a DB25 parallel port, the situation becomes this:
Code:
DB25 pin 1 (strobe, in/out) = NES controller pin 7 (VCC/5V) (white)
DB25 pin 7 (data 5, out) = NES controller pin 1 (ground) (brown)
DB25 pin 10 (ack, in) = NES controller pin 4 (D1/data) (yellow)
DB25 pin 13 (select, in) = NES controller pin 3 (OUT0/latch) (orange)
DB25 pin 19 (ground) = NES controller pin 2 (CUP/clock) (red)
...which doesn't make much sense at all. So, parallel port is probably out. It could still certainly be for something custom, however.
For a new-user first-post thread, you aren't being particularly helpful in tracking down the answer to your own question.