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View of the forward end of the first stage.

The teardrop-shaped holes are blast ports: To minimize the amount of time that the missile was unpowered (and therefore potentially unstable), the second stage began firing while it was still attached to the first stage (referred to as "fire-in-the-hole" staging). These ports allow the second stage engine exhaust to vent.

This can be seen starting at the 26-second mark in the Camera Records Missile in Flight 1963 Newsreel YouTube video. (Although, strictly speaking, it is the interstage, rather than the first stage, that is left "in shattered pieces."