[Ordtech]

Medalguy is right: M2 Ball is M2 Ball is M2 Ball just as M80 Ball is M80 Ball is M80 Ball. Otherwise it would be something else. Our military is not so dumb as to issue a high pressure round with the same designation as the standard rifle round.
The Lake City 30-06 ODCM was selling before they totally ran out of LC was delinked M2 Ball. It was in the description and I could see the scratch marks on what I got.

 

[Ordtech]

Could poor storage be responsible for the mighty bang I got from two rounds but no from the other.

As powder ages or breaks down it gets weaker. Poor storage (high heat/heat cycles) causes the powder to break down. So the answer is no. I think you're right in assuming he had something other than ODCMP ammo. He must have had either reloads or commercial stuff mixed in.
Enjoy your M1. ODCMP has HXP (Greek) in stock at 50 cents a round - $120 for 240 rd can or $96 for 192 rd in 8 round en-bloc clips. The bullets do attract a magnet (NOT armor piercing or steel core, the jackets are gilding metal over steel). HXP Pyrkal was issued at Camp Perry. Safe in your rifle, not allowed on some ranges. Check the rules where you shoot.
You can also get American Eagle with the "M1 Safe!" logo or Hornady M1 specific rounds commercially.

 

[Ordtech]

Some primer sealant looks purple and I've seen some that were black. There's no significance to it other than maybe what contract for sealant at what time or maybe different lots of the sealant changed color differently as it aged. The bullets were sealed with an asphalt tar substance.
Strange situation. I don't know what to make of it other than break down any rounds you have left and reload them with known powder.