Hello, here's my USM1 helmet.
The Inland liner is Early WW2, some one tell me it's a rare one due the type of pressure butons for the sweetband.
I also had with the helmet a photo of the person who recovered it in 45 (the photo is dated five years later August 1950) with the helmet on the head. This clearly shows that the T1 chin strap attachment does not come from an M51.
I also attach three WW2 photos with this T1 system.