Lenovo ThinkSystem SR570 FAN Option Kit

https://lenovopress.com/lp1047-thinksystem-sr570-server-xeon-sp-gen2#cooling

The SR570 server supports up to six hot-swap system fans that provide N+1 cooling redundancy. SR570 server models with 4x 3.5-inch or 8x 2.5-inch drive bays use single-rotor fans: Models with one processor include four system fans, and models with two processors include six system fans. SR570 server models with 10x 2.5-inch drive bays include six dual-rotor fans for models with one or two processors.
Configuration note: The SR570 FAN Option Kit (4F17A12352) includes two single-rotor system fans that are required for field upgrades that add a second processor to the server models with 4x 3.5-inch or 8x 2.5-inch drive bays. If two processors are selected in the initial server configurations, two single-rotor fans for the second processor are derived by the configurator.