Re: type of haversacks used, & if markings were us
"I have located two types of Confederate haversacks"
I assume they are reproductions?
"one has a button-down flap, & the other has a belt/buckle fastening"
Another type of fastening was a simple string knot. Officers haversacks were typically better made with leather straps and buckles. There were many different types and makes, I don't think there is a "correct" one. The most common towards the middle to end of the war would have been Federal issue.
If I were doing a common Confederate private, early war, I would go with a simple canvas white duck with two buttons, either pewter, wood or bone. But find a photograph and try to copy the example in the photo if you want to be exact. Some were stamped in the inside with a company name.