Thank you for asking, Larry – I had to look it up:
Yes, this Great Helm did exist. It is based on an image contained in the Codex Manesse of 1304 (updated 1340). Further, the knight is depicted (imho) as a knight of the Teutonic Order. In short: Matt and Constantin got their research right.
You might remember from the class on Early German Poetry during Elementary School that the Codex Manesse is a compilation of Germany’s best poems/poets starting on the top with royalty and ending with commoners. A number of these gentlemen had somewhat top ten hits. Herr Walther von der Vogelweide comes to mind who was then later featured in Tannhaeuser from Richard Wagner.
Having said that, the wearer of the Grand Helmet in the Codex Manesse is identified as Walther von Klingen.
https://en.wikipedia.org/wiki/Codex_Manesse
https://www.pinterest.com/khaentlahn/1304-1340-codex-manesse-images-only-zürich-swabia-/
https://de.wikipedia.org/wiki/Walther_von_Klingen
According to Wikipedia (too lazy to go into primary sources), Walther was born in 1240, a good 50 years after the Battle of Acre (1189). The helmet might have existed before Walther was born of course, but I would take the position that this Great Helm did exist and in this case is just used as an example as a helmet that was born by a member of the Teutonic Knights.
There has always been a discussion whether 13th century knights really wore these elaborate helmets in battle (where every pound counts after several hours of fighting for your life) or whether they were reserved for competitive jousting where you had to impress judges and the ladies. In the Codex Manesse, Walther is shown in a jousting game which would indicate that the helmet was not worn in battle. I have not done my homework on this and therefore cannot tell you whether the Teutonic Knights did actually use helmets like this in battle, but from the little I have read the Teutonic Knight actually did wear them as a means of identification on the battlefield.
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