These bayonets have always been very difficult for me to correctly identify. Here is some information that was provided by Reino Karha on the subject:
M27 ( Army )
Blade is replaceable and attached to tang by means a dove tail joint and rivet. However this does not show outside. Grip rivets are domed to grind them easily off to strip the bayonet for repair. Crossguard has heavily rounded sides and is loose and hold on place between blade and tang. Pommel and tang are made in a single piece.
Blade sides are round. Crossguard and handle are blued, blade is bright.
Original M27 was in production very short time. Most likely all made bayonets vere modified later on and I have myself never seen an original M27.
Modification was done in 1929, in factory drawings they call it M29 but in Army nomenclature is remains as M27.
The first picture actually shows M29, M27 looks similar but it does not have the spot welds behind crossguard.
There are no other markings on these bayonet than maker's name on blade and serial number which in Army bayonets normally is on top of tang.
M27 was made only by Fiskards, M29 both Fiskars and Hackman though majority by Fiskars.
Scabbard is blued, fluted model.
Variations:
-Originally there is no oil hole on pommel but some bayonets have it drilled at later overhaul.
-Tubular rivets vere used in later overhauls.
M28 ( Homeguard )
Blade is similar as above with round sides. Blade and tang are velded together at middle of the hilt. Weld seam can be seen in most cases. Crossguard sides are straight and it is attached on blade by 2 pins behind crossguard, inside the grip. As there was no need to strip the bayonet in parts for blade change, grip plates are attached with flat ground rivets. Crossguard and handle are blued, blade is bright.
M28 were made only by Hackman & Co. Other markins are -S- or KE in a circle on crossguard, serial number usually below tang, large S in front of serial number and 2 digits on pommel ( last digits of the actual rifle serial number ). These features appear only in Homeguard ( SkY ) bayonets.
Scabbard is fluted and lacquered green.
Variations:
-Originally there is no oil hole on pommel but some bayonets have it drilled at later overhaul.
-Grips have screws instead of rivets.
-Tubular rivets vere used in later overhauls. Sometimes the bayonet was reblued overall.
M28-30 ( Homeguard )
All metal parts are bright. Blade has clearly visible sharpening edges. Crossguard is attached by 2 pins thru crossguard. Blade is weded to tang, not in middle as M28 but about 1/3 from pommel. Curled birchwood grip plates are attached with screws. Oil hole on pommel.
Majority of m28-30 was made by Hackman & Co. and less than 1/3 by Fiskars. Other markings areSk.Y. above maker's name on blade, -S- on crossguard, serial number with S below tang and 2 digits on pommel.
Scabbard is plain and blued.
Variations:
-Without oil hole on pommel.
M35 ( Army )
Quite identical to M28-30 except: All metal parts are blued, grip plates are of normal birchwood and dark.
No other markings than maker's name on blade ( Fiskars or Hackman & Co. ) and rarely serial number on tang top.
Blade tip in Hackman made bayonets is similar to M28-30 but in Fiskars made more sharp.
Scabbard is plain and blued
It's a pain to write it short and understandable but I hope this helps at least someone to ID his bayonets
One addition as this was asked me: Army and Homeguard ( Sk.Y. ) had their own bayonet models as described above. They were not used crossover before WWII when all possible bayonets were mixed - though not newly marked.
Thus you can't find Sk.Y markings on Army models or vice versa.