Cisco IOS 15.7(3)M8 is part of the 15.4M and 15.5M technology train. For a 3900 series router, this is often considered a "gold standard" release. Here is what it brings:
Cisco ceased releasing bug fixes and security updates for the 15.7(3)M train in late 2020 or early 2021.
: A "Universal" image containing all features (Security, Unified Communications, Data) that are unlocked via Cisco Software Licensing. The "k9" indicates it supports payload encryption. mz : Indicates the file is memory-resident and compressed. SPA : Digitally signed by Cisco for authenticity.