I Love LP Steel Tanks because they are full at 2400psi, and you can overfill them which gives you more (for ex: a LP 121 cu ft overfilled to 2650, has 131 cu ft of Air), ..........
I believe LP tanks are marketed at their "plus fill" rating. Your 121 is a 121 cf at 2640 psi, 109 cf at 2400psi.