Hello!
I have seen a lot of genuine Phrobis M9s, issued to the troops, with "storage" markings on the front/back of the blade and scabbard and even on the pommell.
These markings were made directly by the armorers, as far as I know, and usually they have a letter and a serial number.
You can usually find them engraved by laser pen or simply by marker (in different colors).
The marking "A08" could be one of those.
I have personally never seen a marking like "AO 8545", with two letters and a four digit number.
Why don't you post a picture of the bayonet or a link to the auction?
P.S. The markings by the armorers are usually applied "by hand", so they are easy to spot, if you know what I mean.
Look, for example, at the third picture in this auction
http://cgi.ebay.com/EARLY-M-9-BAYONE...item1c187ff8c1
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