Actually it is for both WM5 and WM6. I used MSI extractor and there are four flavors in it for the US english version:
office_smartfon_WM6_131.CAB
office_smartfon_WM6_96.CAB
Office_wpc_WM5.CAB
office_wpc_WM6.CAB
I assume that for WM5 there is only one choice.
It does allow to install to SD card so hopefully it will not impact as much
Jeff