The M9 Bayonet traces it's history to a small R&D company called Qual-A-Tec. In brief, the company was created to support Special Forces and the Rambo movies were just out so they developed a knife which would later become the Buck-184 Buckmaster. In order to deal with the licensing of this knife to Buck Qual-A-Tec created the company Phrobis, Ltd. A few years later, there was a request for Proposal from the US Military for prototype bayonets and the Qual-A-Tec folks took the Buckmaster knife and modified it to become what we know as the M9 Bayonet today. Qual-A-Tec went on to establish a new company to handle this knife and they named this company Phrobis III, Ltd.
Buck manufactured the first M9 bayonets for Phrobis in 1987. The first contract was for 315,600 pieces with a couple of 60,000 unit options at military discretion. In total, Buck made about 325,000 bayonets between 1987 and 1989 for the US Army. Standard USGI Phrobis M9 bayonets are generally classified within 4 unique generations and all follow the same original specifications in their construction.
Gen I: Has 3 lines of information stamped into left ricasso of the blade. Line 1: "M9" with a "flat-top" M and the "lazy" 9, line2: "PHROBIS III", and line 3: "U.S.A.>".
Buck, without the knowledge of Phrobis III, added their date code to the first 1200 bayonets. This was not in keeping with the Technical Data Package and Phrobis instructed Buck to immediately remove the mark from any subsequently manufactured bayonets for the military. The majority of the chevron marked M9s had been shipped before the mark was discovered. One hundred sixty-six pieces of the 1200 piece run had not been shipped and these were held back. The returned bayonets were sequentially numbered on the right ricasso with the odd numbered bayonets going to Buck and the even numbered going to Phrobis. Buck sold their bayonets to members of the Buck Collectors Club. Phrobis manufactured an oak presentation plaque with the front end of an M16 rifle attached to it and sold the plaques to collectors as well as presenting them to dignitaries. These bayonets, like all the Phrobis military contract bayonets, had the Phrobis markings on the left blade ricasso and the right ricasso was unmarked.
Gen II: Also has 3 lines of information stamped into left ricasso like the Gen I, but does not include the Buck "chevron" mark. Line 1: "M9" with a "flat-top" M and the "lazy" 9, line2: "PHROBIS III", and line 3: "U.S.A.".
Gen III: Has the same 3 line stamping on the left ricasso, but the "M" and "9" are stamped in a different font. The "M" now has a pointy, or sharp angled top of the letter, and the "9" now has a curved upward lower portion.
Gen IV: Has a 4 line stamping on the left ricasso using similar fonts to the Gen III version. However a fourth line has been added with the wording "PAT. PEND."