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The CMoS is widely known outside the US, the GPO manual much less so. (At least that's my impression or opinion; any other votes out there?) I would guess that they're equally 'Americanized' and I don't think that's a bad thing. Non-US English speakers are a lot more used to reading US English than US readers are to seeing international English(es). So if your company is writing for a global audience, US English will raise fewer eyebrows, I think.
I haven't seen the new edition of the Microsoft Manual of Style but from the table of contents (http://shop.oreilly.com/product/0790145305770.do) it's quite thorough and up-to-date and would cover a lot of issues that CMoS might not.
Why not have a small set of references for different purposes, for example:
Primary reference: (your company guide)
Spelling: (name a dictionary)
Web content, publications and product UIs: MMoS 4th edition
Other style and usage issues not covered by MMoS: CMoS
Stuart
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