[Linked Image from i.imgur.com]

Drew , the STD stands for Standard Nitro Proof rather than Standard Steel. As this chart from the current Eley Shooters Diary shows it is just the normal Nitro Proof level (for lead cartridges).

Superior Proof is again for lead cartridges but higher pressures.

The Steel shot mark is the CIP Fleur de Lys Steel shot mark.

In the UK standard steel shot cartridges are supposed to be usable (subject to maximum shot sizes and minimum choke sizes ) in guns Proofed for Standard Nitro.

Guns Proofed with the Fleur de Lys mark should be OK with steel loaded to CIP Pressures.

BUT I understand many American steel cartridges are loaded to higher pressures allowed by SAAMI but in excess of CIP limits.