I don't think Krupp had anything to do with it and it is some Russian steel variant, which furnace, I can't say. I've looked at a few examples some with the rings, some without and there's nothing I can see that relates back to Krupp. SpeZial Gewehr Lauf Stahl was Krupp's and this type steel was to mimic Krupp's. The phrase is odd in that it mixes English & German. I can't find any reference to 4 Rings for Krupp.

Kind Regards,

Raimey
rse