A rather obscure construction detail that is not commonly used is the continuous fly. There are relatively few new jeans that uses this. Certain models of Warehouse Duck Digger and Rising Sun & Co. come to mind (the former uses a selvage continuous fly while the latter's Blacksmith jean does not).
Below is a clipped image from the United States patent granted to David Neustadter on October 30, 1877. It is widely known that the Neustadter Brothers in San Francisco manufactured the "Boss of the Road" overalls. His intention was to patent a fly construction method for overalls that was stronger without necessarily adding bulk.
Because the fly piece itself does not have seams (held by cotton threads) that can rip it is considered to be stronger. On most modern jeans your "first line of defense" is usually the stitches that rise from the crotch seam, which are generally strong enough to obviate the need for a continuous fly construction. But as a denim nerd you may want at least one pair of jeans with continuous fly for bragging rights. If you are interested enough you may want that pair to be a vintage Boss of the Road.
Below print is from the the July 21, 1899 issue of the Victoria Government Gazette, which printed various legal notifications and publicly declared trade mark and patent applications in Victoria (Australia). As you can see Boss of the Road's marketing images proudly indicate "with patented continuous fly."
Victoria Government Gazette