With expanding blades
According to Wyndham Powell, writing in 1909, Kollmann dialtors should be used with rubber covers or sheaths.
See "Wyndham Powell BMJ 1909 A point in the use of Kollmanns Urethral Dilator".
The dilators could be either curved or straight, but both had four blades which expanded to a calibre determined by winding the dial (bottom image).
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