That is a 72-4 boot. 71 is a rubber double accordion style.
I'd agree with you that's the norm...but I've seen two 71 GTX's with bench seat 4spds over the years (fairly looking original cars) with the leather boot....original?, GTX only?
But regarding the repro of the leather boot, I was looking for a repro a few years ago, nothing found, I haven't seen or heard of anything of late but you never know if someone out there has made a small run?.... the OP might be better off on the 71/71-4 B Body only Facebook forums to find one or an individual making them, you'd be surprised at what's out there being made/offered...