The "SP" means it has been proofed for steel, but Beretta doesn't recommend using anything tighter than Mod with steel. Seems odd that they would say it's proofed for steel but don't use it, but what do I know? Here's the direct quote from the Beretta site:
Can I use Steel Shot with my Beretta Choke Tubes?
We suggest that - when using Steel Shot - you only use choke tubes marked with the SP (Steel Proof) identification on the side and that you use the most open choke system possible – i.e. Cylinder / Improved Cylinder / or Modified. We DO NOT recommend use of Steel Shot in the X-Tra Full Turkey / Full / or Improved Modified choke tube."