To answer the questions, the "pins" are small, forged tacks that hold the base in place. Being 3/8" thick, there is plenty to "nail to." I have seen a couple of other original horns with leather base plugs. It is thick stuff and of course if you cut it to a very tight fit, wet it, and then place it in the base, it can make a pretty good seal on its own.
The hole for the spout plug keeper string is drilled through and knotted on the inside. I sealed it with wax and it does not leak. Again, I have seen this on a couple of other old horns. It does require maintenance to be sure it stays sealed, but the hole is very small.
Thanks,
Jeff