A few facts to set this up:
1) I am writing an ongoing series that's set in 1975. Because of that, I've done a lot of research to remind myself what boys clothing was like back then (and if you want a really good resource to check clothes from a number of eras, this site has catalogues from several different major department stores many, many years):
https://christmas.musetechnical.com/2) Some of you probably know that I have a bit of a fetish about tighty whities, so it should be no surprise I did a lot of research on this topic.
I present my findings here in hopes that it's useful to others who might be writing period stories, and that someone might answer any questions I've left, or correct any mistakes I make.
Thanks.
From what I can tell, boys underwear fell into two main groups - labels and store brands. By labels, I mean those companies who produce underwear and sell it in my different locations. Of course there are the big three - Fruit of the Loom, Hanes, and Jockey, but there are several others - BVDs, Carter's, Munsingwear, and Stedmans. (I think some of that latter group might be more regional, but I'm unsure).
BVDs -

Carters -

Fruit of the Loom -

Hanes -

Jockey -

Munsingwear -

Stedmans -

It's unfortunate, but it's very hard to find information about underwear of different years. Most of these pictures come either from eBay auctions or advertisements in relevant years, but in some cases, nothing was available, so I had to stretch.
With most of these, you can easily tell that they are boys briefs, but it's harder to be sure of the time period.
When it comes to Munsingwear, I couldn't find anything from the right time period or for boys. However, I shared this picture, because Munsingwear offers the unusual 'kangaroo pouch' fly.
As for Stedman's while I couldn't find anything 70s era on boys briefs, the ad (which is from the 50s) seems to show that the boys and mens briefs have the same waistband markings.
I'm going to address the store brands in a separate post. In the meantime, if you have more information, correct information, or just want to talk about one of these brands, please feel free.