Good job.
I'll recommend another book if you think you'll stitch more leather.
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A regular 2 needle saddle stitch is very doable without a pony or clamp, just slower. But, you can pull those stitches much tighter.
Weaver leather sells a great little stitching pony for around $18.95 or so.
I glued some thick leather, rough side out, inside the jaws to give it a little better grip.
If you don't want to mess with some type of button to help keep the flap closed you can imitate a flat sewn flap with one more line of stitching just below the fold. Effectively stitching the flap to the back panel like this,
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I think you bag came out really nice.
If you decide on an antler button, remember that these would not have been sliced into rounds and holes drilled through the middle, the middle is pithy. Properly made would have used the side of the antler as the outer surface,
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I doubt the bottom "stud" type button would have been made and was a royal p.i.a. to make anyway
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Metal, bone, wood, and cloth buttons were far more common.