Looking at the pic, there are a few things that contributed to the "hamon"...... It's a pretty safe bet that the blade was a bit on the cool side when quenched, and the thinner cross section near the edge is all that was through hardened. Combine that with the fact that it was "mystery" steel, and just about anything could happen.
Something I've come to understand in attempting to help others is the fact that most don't realize that there is a difference between heating steel to a given temp, and having the steel at the temp when it hits the quench. I worked with one individual for a couple of months, before it occurred to me to ask him where his quench tank was in relationship to his heat source.... "About 20 feet away" was the answer he gave me. He was heating a blade, then walking 20 feet (give or take) to his quench tank.... no doubt the steel was well below temp by the time he got there and quenched a blade. Little things that might seem insignificant do make a difference.
As Doug mentioned, Rc testing it will be near impossible to do accurately unless the surfaces are parallel.
The biggest bane we face these days with using "recycled" materials is the fact that nearly everything has gone to "spec manufacturing"..... In short, that means that specifications no longer call for a particular type of steel for an item.... now a days the function is stated, with the caveat of "use the cheapest material available". That means that the exact same item, from the exact same company, could be any number of different steels from production run to production run. If folks choose to use "recycled" steel, then it's just a constant guessing game.