Many industrial, military, and healthcare networks still run legacy hardware (e.g., Cisco 2600 series with serial interfaces). Finding a specific IOS 12.4(15)T image for a router that hasn’t seen an update in a decade is nearly impossible via official channels unless you have a active SmartNet contract.
When enthusiasts or engineers refer to a "Big Collection," they are usually discussing the massive variety of feature sets—such as —that were tailored for specific hardware platforms like the legendary 2600, 3750, and 7200 series. The Architecture of the Collection Cisco.IOS.Big Collection
