Wild

Are we missing a post above or did you suddenly read something that made you go "Yikes! in your post ?

400 relates to the Bullet weight

60 IS LIKELY to refer to the Max Load which is normally
the full load of 60 grains of cordite (their was a 55gn "Tropical" Load as well).

The reason I say LIKELY is because I like to see the proof marks in a photo before saying definitely.

"proof marks BV and NP if that is significant."
That's an understatement.

Your gun looks like it has been proofed for the
full Nitro load used in that calibre.