Hello Colin,
thank you for your question....I saw the auction but I didn't think about it!
First of all the auction link
http://cgi.ebay.com/ws/eBayISAPI.dll...earchZ1QQfviZ1
I have looked around in my notes, and I found part of a letter by H. Brett which could help you.
Here it is:
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Sorry because the image size is not that great.
Basically, from what I understood, while the military contract ended in 1989, an agreement between Phrobis and Buck was arranged only by April 1991.
So yes, in 1990 you could still find the Phrobis&Buck marking on the commercial M9.
Hope this helps and apologize for my english!