Hi John,
I used to have a nice Bavarian M95, which was stamped 13IR, but in ink, not pressed in the helmet. And, if you recall James had 5 helmets for sale here
on the forum, one of which, a Bavarian too, an M95, was stamped exactly the same as your helmet, 18IR. Pressed in the leather.
https://www.pickelhaubes.com/bb/viewtopic.php?f=6&t=14957&sid=024161da9f1fb27d1c0ead787a2c7df5
Helmet #1
So I'm inclined to believe it say's 18 IR. I saw this more when visiting collector friends of mine, but took no pictures..
Greetings, Coert. (by the way, I bought a nice M15 Bavarian today, will post pictures when it arrives here).