As for the differences in material and such. There are three answers.
1. The specs or follow on correspondence from the Army sometimes stated substitute material that was allowed to be used.
2. The army inspector and company worked out a substitution that both could agree on- often when there was a shortage of the item the inspectors could have more leeway, as well as if there was a stock of different material that might otherwise go to waste.
3. The company used a substitute and it was not OK'd by the Army, so it was never actually accepted, and ended up in a warehouse and being sold off after the war as surplus (even though it wasn't), or taken home by employees, or whatever.
That's why you see things like pocket material be different- it didn't matter much to the army what the unseen material was, and if there was surplus of something they were not going to use, the inspectors could decide it was just as strong/good and save the Army time by not waiting for the right stuff, money by using material on hand that would not mean more transportation and mill work.
The army may have been picky about their stuff, but they weren't stupid, and with a war on they did try and conserve.