The trousers construction (and drafting) method from Clarence Poulin's book Tailoring Suits the Professional Way. The first file is from the 1973 revised edition and details the zipper-fly method:
Zipper fly Method (https://www.dropbox.com/s/8772gisn8gz752y/Poulin%20Trouser%20Making.pdf?dl=0)
The second file is the button-fly method from the 1952 edition:
Button fly method (https://www.dropbox.com/s/nm56wz14627myqv/PTM-1952.pdf?dl=0)
Poulin writes in his book:
(http://i628.photobucket.com/albums/uu5/vanderloo/poulin-text.jpg)
Both methods are worth tackling.
https://archive.org/details/pdfy-6x5RRDlrC8nRAeWc
This is a link to the complete 1973 version pdf.
This book is a great help when you start making coat.
It help me a lot in my early days.
I'm happy it's on the net, it's to expensive to buy nowadays.
Yes, someone did a good job of putting that on the archive. I put only the trouser parts here so that the apprentice area would be free of coat topics. The whole book is probably better placed in the reference section?
I have the entire 1952 edition as well.