"The Clue of the Velvet Mask" was book number 30 in the original Nancy Drew series and first published in 1953. The fronts-piece illustration for the original book is above. This also has quite a bit more violence than your "run-of-the-mill" Nancy Drew story, especially George's kidnaping scene (for which I've never seen an illustration for and, I doubt if one exists.) I've chatted with ND enthusiasts who say this book is sort of a "holy grail" for the bondage illustrations. Yes, you read that correctly, illustrations. From the original cameo version (of which I've included here, though heavily reconstructed from poor scans), to the later version, both illustrations are, indeed, actual and not part of some "bookly urban legend". The above illustration occurs just prior to Nancy and Bess getting captured. The two are taken into the damp, dark basement of the inn and left bound, gagged and blindfolded. They are later tossed into the back of a lorry, which then proceeds to the department store (though the two girls have no idea of where they are headed.)
In an early scene at one of the extravagant parties the crooks like to frequent and rob, one of the maids relates to Nancy how she was grabbed from behind and left bound, gagged and blindfolded. It's a recurring theme in the story. There is no illustration for this scene that I'm aware of.
The second scene involves George's brutal kidnaping. I'm using the original set-up as it was heavily edited later on. Nancy has one of the velvet masks which the crooked gang uses to hide their faces. She's at home when she gets a telegram (soon to be revealed as a fake), from her father. She's instructed to meet him in another city and bring the mask. Nancy enlists the help of George and Bess and they all disguise themselves with wigs, with George acting as Nancy and carrying Nancy's monogrammed suitcase. Later on the train, the woman of the gang spots the suitcase and assumes George is Nancy. Here's where things get hectic. It is not revealed how in either version but, George is knocked out. Clews would indicate chloroform. The incident happens just as the train is pulling into the station. One of the two other men block Bess and Nancy's path as George is picked up and carried off the train. When the two girls finally manage to alight, George has already been shoved into the back of a waiting car, which speeds off. George is mostly out of it as she's interrogated by the woman. Due to the heavy amount of the drug used, George can barely talk or move. She's tightly blindfolded. One of the crooks says he carries a "potion" that will get her to talk and George soon feels a "sting" on her arm. This would indicate use of a hypodermic needle, although it's not directly alluded to. This whole scene is eliminated in the revised edition. The woman whispers dire threats into George's ear (which are not revealed) and she's soon left abandoned in a nearby wood. Meanwhile, Nancy and Bess have tagged along with the state police in the hunt for George. Nancy finds her first and George stumbles toward her, crying (uncharacteristic of George to do this.) Furthermore, George is so put out from her experience that she doesn't tell Nancy about what's happened other than for her to drop the case.
Nancy's first tie-up scene happens at a costume party. Nancy is accosted by the woman thief up in one of the bedrooms. Her partner strong-arms Nancy and tightly wraps her up in the bedsheets. He then piles a lot of heavy coats on top of her. The thieves lam and Nancy struggles to get free ere she suffocates. The original version is below.
Further below are the scans from the cameo version of this book, which shows Nancy & Bess bound, gagged and blindfolded (in the newer version they are not gagged in the illo but are in the story.) There is also the "escape attempt" in by which Nancy finds a chemistry set inside one of the packing crates and puts it to good use. The girls find that they are being held in the shipping/receiving part of the department store. In the story, Nancy's wrists are bound so tight that the ropes cut into her flesh. Oddly, her feet are not tied as tightly. In the darkness she gropes round and finds a sharp piece of metal (possibly part of a strapping band but not alluded to), and manages to cut herself free. She finds a pull-cord for an overhead light and turns it on, then goes looking for Bess. In the illustration, the two girls are together but, the story has Bess some distance away in another part of the room, hidden amongst the packing crates.
In the latest version (mine is from one of the Nancy Drew "double books"), I've arranged the illustrations in the order in which the scenes appear. Note that the woman is prodding Nancy with her left foot. This occurs in the original version only.