I'm no expert on DIS (dual-plug ignition system) but I believe they were introduced around 1991. I'm fairly sure they weren't available in 1987. Someone probably swapped the later motor into your '87. But then, I surmise, they would've had to swap in the electronics also - computer, wiring harness, etc. I suggest you go to
www.turboford.net or
www.stangnet.com and ask over there. They're much more up on the DIS motors than I am.
Oh yeah, dual plugs means twice the spark, thus a much better burn of combustion chamber gasses which equates to cleaner emissions. Maybe even a slight increase in power, too.