Version 5.1.5621 is compatible with the following Mac models (non-exhaustive list):
While Apple officially says these older Macs don't support Windows 10, many "helpful stories" online detail how users used the 5.1.5621 drivers to get Windows 10 running smoothly, manually running the in Compatibility Mode. Apple Discussions Supported Models for 5.1.5621 boot camp support software 5.1.5621
Here is what fixes that newer versions break: Version 5
