Well, as you might know, private purchase helmets are mostly of better quality than standard issue. So it's normal that they were made with better (looking) materials and very often made in officers quality. I suppose that Einj. Freiw. sometimes even had more than one helmet, one for every day use and one for "official use".
In this case we see that the chinstrap is attached with the officers rosettes; there has never been anything else there, so originally made like this. You can also see the imprint of the chinstrap in the frontvisor...
The vented spike is of course also original.
The rear spine is unvented.
So in my opinion it's a high grade helmet wich was very expensive to purchase...