My Maxum has the "fleur de leys" proof mark for high performance steel shot. So I am assuming yours will too.
Have a look on the barrel lump...
On a side note, in the BASC website there is a whole article about this, and it states that regular steel loads are ok in pretty much any nitro proofed gun up to half choke.
You only need the "steel proofing" for high performance hardened steel shot. Have a read below...