Cecil once again brings us another enlightening piece that inquiring minds want to know more about. 
I understand how a small slit was put in on models that still allowed the women to urinate without allowing intercourse or even for a woman to touch herself. He mentions also that metal teeth were sometimes put around it. But I’m hoping he could elaborate on this part:
In the duplex models, you had a somewhat larger aperture in the back to permit defecation while preventing anal intercourse.
How is that possible to have a larger aperture in the back that allows for defecation while preventing anal intercourse?
John
An opening about the diameter of a quarter would permit the passage of feces but not a normal-sized penis.
Link to the column.
http://www.straightdope.com/classics/a1_072.html
[waiting for the inevitable “what is a normal-sized penis, anyway? What about a dime? What about a nickel? What about a Kennedy half-dollar?” hijack to begin…]
Link to column
I’m not sure, but Cecil does mention that use of the facilities would have been rather messy. Some possibilities which come to mind are an undersized opening, or one with wire crosshairs, through which fecal matter would need to be extruded, or a bent passageway of some sort through which it would need to be worked in stages.
One might also have various active defenses, so to speak, in the posterior of the device (one has a mental image of “severe tire damage” strips), but knowing Cecil’s sensibilities and writing style, I doubt that he would have missed the chance to mention such contrivances again, were they present.
I wonder if the chastity belt thing is very much a myth. Some accounts suggest the quality of metal hundreds of years ago was not conducive to long-term wear of such an object. Corrosion would have meant they became intolerable after a short time.
If I remember correctly Alex Comfort in Joy of Sex suggested that if they were worn at all it was only for short periods and solely for protection from rape. Women must have lived constantly with the fear of rape in ages past. Any history book will give evidence of that. I assume that if they worked at all it was to stall for time and increase the chance of rescue.
I wonder if a modern version, made of, say, titanium would work - it could maybe have a little alarm attached. Could a woman walk through L.A. central on a dark night if she wore such a thing? Rape is just about violence, I know. Maybe it would make things worse in the long run. I like to go out and look at the stars at night. I’d try one.