Not just a Jensen #50 whistle modification, but that same set-up will modify the whistle on just about any other Jensen boiler as well.
I've always been sort of curious why Jensen never developed a better whistle, but then again, the standard Jensen whistle always overloads and shuts down the sound recording on my videos, so I must assume that it is actually a very loud whistle at a frequency that I can't quite hear, but my camera can?!?!