It's not just the stamper number or where it was pressed. It can also be the time of day it was pressed. As I discovered with two copies of the Music Matters 45rpm reissue of Horace Silver's, Song For My Father. One copy had a hot high end, bring the cymbals forward and highlighting the initial bite of the trumpet player. As a result, that copy was emotionally engaging, having me sit up in my seat when listening. My other copy sounded sort of laid back. The top end was not nearly as hot. The cymbals have less of a shimmer and less air. It threw the soundstage farther back and I don't feel as compelled to sit up at the edge of my seat while listening. MM's press up to 500 per stamper, so they're certainly not wearing out. But why the difference in sound? I discovered that the water cooling temperature varies depending on the time of day, and that can alter the sound as well.