Nice find!

As others have said, your MP ammo is highly collectable, especially a full and complete box.

Technically your ammo is not "armor piercing" but some states may consider it as such (check your local laws). It has a lead core and the jacket is thicker at the meplat (tip) of the bullet.

Check this link for a photo of a sectioned bullet of your round and you can see the noticable difference in the thickness of the jacket:

International Ammunition Association {iaaforum.org} - View topic - 38 Special Western Super X Metal Piercing