The mysterious M51, M72s Bulgarian helmets. Part 1

[Jack59]

Hello,
This article piqued my curiosity and I went to examine mine.
I would like to present an M51/72 for its color, because it has a mark (54) on the hull and a round stamp whose meaning I am completely unaware of...
I also notice that the first rivets / ventilation holes which are more rounded have 2 tabs and the newer, flatter ones have 4 tabs.

Thanks to post your helmet, so far I've seen 4 type of air vent/ rivets for the M51, the circular stamping seems like an acceptance one & the cold stamp is a first for me. Never a dull moment with Bulgarian helmets! Once our Bulgarian members take a look maybe they will provide us with answers. Meanwhile I'll dig a bit because that O.T.H. rings a bell.
Jack

[Jack59]

Meanwhile here is an older helmet with 4 tabs.

[Jack59]

M51s & M72s seems to have been issued to mechanized troops,
specialized trades (signals, artillery...) NCOs & Officers.