AFAIK, the expression "Spend a penny" was because women have always had to pay to use public lavatories.
One of the things which has changed in my lifetime has been the willingness of young women - usually when in a group out boozing - to pee al fresco.
I'd not heard of anything like Pete75's post about the leather thong on a police staff being used for flicking, but as I must have posted before, in pre-CPS Leeds it was treated as disorderly conduct or more usually drunk and disorderly.
This type of thing sometimes shades into indecency. In my plain clothes days (1971,) I dealt with somebody for exposure when the explanation from his mother of all people was that it must be something to do with extraordinary length, which she said he had inherited from his late father.
When I recounted this to other colleagues, one told me a tale from years before that when a flasher who was proud of his endowment was invited to lay it out on a police station table for measurement and received a sharp tap with a heavy ruler.