A lot of reenactors used Spanish helmets as they were affordable and wouldn't cause harm to a real Germanicon helmet. Quite possibly how the helmet I saw found it's way into an antique shop. The dealer may not have any idea it isn't real and overpaid for it himself.

You have to be very cautious with these things. I found an M34 fire police helmet yesterday that i considered buying but I cannot verify that it is a WWII item. It does not have standard helmet markings and the liner does not look the same as examples I can find. It's in good shape, the price isn't bad if it's real but I am going to pass on it for those reasons. They made them into the 60's and there are numerous variations of both the WWII ones and the post war ones and unfortunately there does not seem to to be a website that specifically shows how to tell them apart.