About the "RED" photo up :
CUTTER PLATE MARKING " # 3 "
Courtesy from PWCOSOL A.K.A Fred, (U.S.A.), great M9 bayonets collector
My understanding is just as the US Army XM9 trials concluded, it was realized the Phrobis submission lacked a blade stop.
Without some form of stop, when the wire cutting feature was used, the cutting edge of the blade could come in contact with the user's hand, with painful results.
Phrobis then fabricated a very small number of 'product improved" M9s which incorporated the now familiar "button" stop found on the Buck 188 models.
I think this is when the change in angle of the lug on the cutterplate also occurred. In addition a warning sticker was also to be applied on all subsequent bayonets.
The length of the rear cutting edge on the bayonet was also reduced so the sharpened blade would never make contact with the user's fingers.
It is assumed the Army was unwilling to incorporate such design changes in the finalized M9 version they accepted (recall how they complained merely about the ">" date code added to the first M9s delivered!). Buck then utilized the modified cutter plate & sticker on their commercial product, which if for not any other reason, might protect them from any potential product liability claims. This seems evident from the letter provided by Buck with the first M9 bayonets they offered to their Buck Collector Club members (the withdrawn Phrobis "three-line chevron" M9s).
The letter states "It is not safe for civilian use without changing the enclosed cutter plate (with safety stop) and installing the enclosed safety warning decal."
Why they never elected to go with the reduced sharpened edge is only known to them.
Note this bayonet has the number "3" stamped on the unfinished cutter plate, and button stop has been added by hand.
Of those few "P.I." Phrobis M9s, some were evaluated to destruction & only a few survive.