It could also conceiveably be a manganese-molybdenum steel alloy, which, if memory serves, is what Reynolds 531 was. I have no clue what the advantages/disadvatages of manganese vs. chromium in steel alloy tubing. All I know is that 531 was the gold standard for a long time and is still good stuff (even though Reynolds isn't making it anymore.) Many builders still used it for fork blades long after the "super steels" had become commonplace.