«In 2020 no collector (let alone a famous one) would sell a rare or even a normal German / Austrian helmet for 150€, so this has my alarm bells ringing.»
My reason is that he didn't actually know what he had. It is a rare model that is almost unheard of, even the MITAG didn't know about it. Also, at first glance it seemed like a normal M16, so probably this is also a factor.
I still have to contact Stefanini tho, one day when I will have the courage I will do so.
Also about the Kaiserjäger photo, you're right. I thought that it was concrete proof but some other collector also suggested that it could have been a touch up.
Anyways, thanks for the help.
P.S. I will also try from now on to search for photographs from WW1 showing the regular M18 in use, seems a fun research.