I would agree with battlegear - telling a 51 from a 51/72 ins't the easiest thing, but the later model does have a slight lip around the rim which isn't present on the original (although I sometimes wonder whether this is actually an illusion...). Apart from that the 72 usually has larger vent lugs and, almost always, sewn-on rather than riveted chinstrap parts. It is not unknown for helmets to be made up of liners and shells of wildly varying vintage, which could explain yours.
Anyone interested in the Bulgars should check this page -
Bulgarian Helmtes' History - but even Ted Muhovski seems reluctant to commit as to what the differences really are.
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