@groom 1) Yes indeed the custom was going into the 1970s but I think it was more covered so people didn't really know it was going on. I remember at my primary and secondary school, a few children with me also turning up sewed into their clothes in the colder months (although of course it was top secret from the other kids!). In fact, even this century about 10 years ago, I remember my half-sister saying that for the Christmas she was going to sew in her children (aged 8 and 14) for the Winter as it was a very cold one that year and she was living in the countryside with not so great heating (I think she had a better time with the sewed on underwear than me and my other siblings did!). Unfortunately me and her don't get along very well, so I wasn't abel to dissuade her...
2) Fortunately, taking a bash and shower were very easy, as the water would just go in-between the clothes and on the skin (the water could just about pass through the stitches into my underwear area and below) and so could wash out dirt. That being said showers were very rare about once a week because my mother thought it better that way for some reason.
3) There was no change of underwear over the 7 months whatsoever, just the same thing for over 200 days (I did try to use deodorant to help the smell though).
4) Yes the treatment was the same for both sexes. For my half-sister, not that I know how she dealed with her pubertal bodily functions, but I presume she just pulled up her knickers and did her business.
@Ayashi 1) One could in fact go to the toilet fairly easily: for a number 1, there was the normal opening in the underwear, and for number 2 it was possible (although with some effort) to lift up the underwear high enough until you could do your business without it getting everywhere (as the stitching was along the top of the underwear where it met the pyjama top, pyjama top tucked into underwear).