To the average player today, this looks like a typo. A minor patch number. A footnote. But to those who were there in September 2010, "1.2.6_01" represents a unique temporal anomaly in gaming history. It is the version that almost wasn't. It is the bridge between the chaotic, infinite Alpha era and the polished Beta era. And the word "Exclusive" attached to it changes everything.
"Minecraft Alpha 1.2.6_01" refers to a legendary "mystical" version of the game that exists primarily within the Minecraft Creepypasta Wiki minecraft alpha 12601 exclusive
If you are looking for "exclusive" versions (like the "12601" number suggests), you might be looking for fan-patched versions or specific jar files preserved by the community. To the average player today, this looks like a typo