For the liner belonging to this shell see USA M1 helmet 1984 liner
The heat stamp: 4692
http://www.warrelics.eu/forum/us-m1-steel-helmet-forum/post-war-m1-helmet-headband-1984-a-707114/
Heat stamp looks like a Parish Division of Dana, late 60's.
A very nice liner considering the age. I think the actual age of the shell would match up closely with the dates seen on the liner. I agree could well be a Parish Division... or perhaps a RJ Stampings shell. Parish produced almost two million shells between 1968-1970. The OD Colour matches well (munsel 10?). RJ Stampings, who I believe were a Canadian company, produced a similar amount of these shells in the early to mid-1970's. Roughly speaking, the distinction appears to be on the size of the numbers. Small are seen to be Parish where RJ are larger, more like WW2 era stampings.
http://www.warrelics.eu/forum/us-m1-steel-helmet-forum/vietnam-era-m1-helmet-parish-division-dana-corporation-capac-liner-655997/
I found aome information about
http://www2.readingeagle.com/article.aspx?id=65605
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