I collect in this area and would like to see more details of your collection. Attached is one that I have not seen before. Can you tell us more about it? This period fascinates me. Chris Dale has posted some remarkable examples of M42's on his web-site. I am currently restoring a Swedish grenadier's helmet. Beautiful plate with big Vasa lions. The unit was sent to Jutland in 1848 in reserve and as a peace-keeping force during a temporary suspension of hostilities.
I couldn't study any of this without Lacarde's pioneering work. Someone could put together a fabulous book or web-site of these. Each helmet has a separate German state history behind it with all kinds of characters, political intrigues, "little wars," and marriages between them that created the first European system.
My favorite is my Schleswig Holstein Dragoon officer's helmet.