I'm going to (politely) disagree with with you and claim that the imprint is more likely "ROD PN 6". Who is ROD? Don't know. 8<)
The vertical support is (almost always) a simple 4" "dumb" pipe made of unrated steel in the cellar/basement. (Like mine, for example.) Scaling, that also indicates the wide flange is a 6 inch nominal size.
Put a caliper (a simple plastic one or small one from Sears - no need for thousandths of an inch calibrated accuracy!) on the (1) beam flange width, (2) beam flange thickness (near midpoint, not edge) and (3) actual beam depth.
You will be able to find the actual properties from a table of nominal sizes when those three dim's match. if any disagreement, throw out flange thickness - it is most likely to be off.
DON'T assume a high-grade steel!