These cuffs include a Hamburg Locking mechanism. The Hamburg-style key is not a standard handcuff key and will only work with this lock. They can be bought in different sizes
Why: They are heavy (1.5 lb) but comfortable and look like jewellery. Imagine them worn with an evening dress. Also, they resemble a metal rope comfortably and inescapably embracing wrists and ankles.
Not sure if such handcuffs can be used for self-bondage (no problems with legcuffs, of course). Probably if the key is clamped in a vice?