The final segment .jpj3 suggests this is not the very first release of 15.3.3, but a subsequent engineering special (likely a rebuild to address a specific issue found in field trials).
: The platform identifier for "Access Point 3rd Generation 2," covering high-performance 802.11n and early 802.11ac hardware. : This is the most vital part of the name. Autonomous IOS ap3g2-k9w7-tar.153-3.jpj3.tar