remote end (bank) is tight-lipped about their setup

The remote software version is the first thing you get when the SSH handshake starts. Just telnet to the remote host SSH port and you will see which software they are running. Running ssh with -vvv also gives you that information.

Here's the result of a dry-run (no files) using debug

This is useless, unless you show us the debugging output of some script where the problem actually happens, we will not be able to help you.