Yes it is an original.
The Helmet began full use as the the M1956 which itself was the adoption into full service of the late war prototype generally referred to as the M44 which saw limited use in the last ditch battles agains t the Soviet Red Army. The original liner was rivetted through the shell, later versions had the same type of liner as yours but retained the three rivets on the slopes of the outside and finally shells were made without the rivet holes but with the attachment studs on the inside like yours. The later shells have a slightly different profile but that is a fairly advanced recognition feature and the liner/rivet/stud configuration is a much easier way to tell untill someone begins drilling and rivetting late shells!!! The generally accepted designation of your helmet is M56/76 although some that look the same have shells dated earlier than '76.
I hope this helps
"War is an ugly thing, but not the ugliest of things. A man who has nothing for which he is willing to fight, nothing he cares more about than his own personal safety, is a miserable creature with no chance of being free unless made and kept so by the exertions of better men than himself."
looks in mint condition so definately a keeper ,have a look for the size and date stamp on the inside shell near to the wearers left ear ,my similar example is dated 86
you have in side size and year (on right side)