One option: pick an arbitrary bank that's been around for a few decades, perhaps one that isn't huge enough to have been investing in building internal engineer

One option: pick an arbitrary bank that's been around for a few decades, perhaps one that isn't huge enough to have been investing in building internal engineering/platform capability for the last decade, but is large enough to have gone through a few mergers and thus internally runs on a mix of legacy IT systems from a mix of formerly independent smaller banks. It may be more mainframes and windows boxes than linux. You might find production solaris machines. Larger older telcos might be interesting as well.