Blizzard operates his criminal empire with impunity having bribed the local police force into turning a blind eye. But the Federal Secret Service has him in their sights. Their head, Lichtenstein (Milton Ross) asks his best operative, Rose (Ethel Grey Terry), to infiltrate the operation for incriminating evidence.
She is accepted by Blizzard, quickly becoming one of his favorites, even coming under his controlling spell. But when she discovers a secret passage down to the cellars leading to a completely outfitted surgical operating room (puzzling) and a large cache of weapons (alarming), she knows she has to report back.
Then ... She scribbles a note and throws it from a rear window to a Secret Service agent waiting outside.
... and Now, the scene above was most likely filmed on a studio set but this window is where it was meant to be - read on for its location.
Then ... the agent picks up the note but he is immediately attacked and neutralized by one of Blizzard's henchmen.
... and Now, The note had been thrown from the window of 9 Old Chinatown Lane above the door at far left below (map) - the window pictured in the Now image above.
CitySleuth recognized this alley having previously researched a foot chase in the 1949 movie Impact which ended up (below) at #11, next door to Blizzard's place.
Here's a 1955 photo of Old Chinatown Lane, formerly Church Alley, viewed from Washington Street. Note that this is a block away from Wentworth Place where we previously learned that Blizzard's office is located. So directorial license prevailed in choosing the rear window site.
... and Now, a fire escape and utility equipment has been added on the left side but Old Chinatown Lane today retains its same character.
Blizzard now knows that she's a spy and she knows he knows when he hints that her number is up. But after commanding her to operate his foot pedals (hmm...) he is swayed by how well he plays with her help... "You're the best pedaler I ever had". Rose is spared.