The original pattern blades had a bright finished blade with the pommel, tang, and cross guard blued, along with approximately 1cm at the top of the ricasso. I have been asked many times the reason behind this small area of bluing, and have seen many explanations as to its reason, it is most likely however that this is purely down to ease of production, in my opinion (the same blueing pattern is seen on a wide variety of bayonet models). The bluing is one of the last stages in production and is carried out after assembly of the pommel, and the cross guard to the blade (even more relevant to the early patterns with quillions). To produce a complete bluing to the cross guard it would be necessary to immerse the bayonet such that ricasso was in the solution. Rather than producing a blade with a little blue line that varied noticeably in width (due to slight and allowable variations in dimensions or liquid height), a deeper immersion would give blued area which disguised these minor variations. We therefore get the blued ricasso from manufacturing considerations rather than design.
In 1915 a change in production was made which resulted in a shot blasted blade rather than the polished blade. Reworked/repaired blades were shot blasted during the process, so early blades may be found with the shot blasted blade. This change was reportedly made to reduce the reflections from the original blade.
1919 saw a rescinding of the change to shot blasted back to polished blades and all new blades were to be produced with the polished finish and reworks/repairs were to be re-polished. This could account for the apparently soft stamping on some blades, which may have been shot blasted and then re-polished. The order was such that issued blades were to be polished by the troops themselves using “the normal abrasive materials”, this polishing was to commence 1” from the cross guard (leaving the marking areas untouched). Blades in store were supposed to be polished upon issue.
In 1923 the blades, pommels and cross pieces were to have a polished finish, removing the original blueing in the process. This change was only for the regular army and territorial units were to be left equipped with the dull finished blades and scabbards.
Although I can find no references in the texts it is possible to find blued and “parkerised” finished bayonets with UK markings, I think these may actually be reworks by either India or Australia, or for resale, rather than for UK issue. The "parkerised" finish is often assumed when the blade has actually undergone sand/grit blasting, as opposed to shot blasting, the finish being similar, Note Parkerising is an applied coating
In Australia in 1938 shot blasting instead of sand blasting was approved, the sand blasting would give a much “rougher” or “sharper” finish than the shot. In 1942 bonderising was added to the acceptable blade finishes.
Most Indian blades will be found with a heavy all blued finish, which ahs been applied to all reworks as well.
US produced blades were finished in accordance with the current requirements for UK produced blades.