$300 for a "Pre-B" CZ 75 sounds like a good deal to me! Congrats.
That "887" marking is actually the Czech proofmark, a lion, followed by the year of birth, in this case 1987. The semi-modern plastic checkered grips and the two-hole safety also ID it as a mid-to-late 80s model, as I recall (those grips debuted in 1987, I believe).
The one factor that throws me off is the lack of the sight rib on top of the slide. In my understanding, the rib first appeared on the original CZ 85, and followed suit onto the standard CZ 75s soon afterwards. CZ has a history of using old stock for quite some time after official changes are made (most famously the "transitional" models of 1993-94).
Can anybody pin point when the sight rib really first appeared on the 75s? I thought it was 86-87, but this gentleman's excellent pistol puts a hole in that theory.
Well, enough of my rambling. You have an excellent gun in your hands, man. You're gonna love shooting with it.