Nam Radio Receiver for M1 Helmet 1967

I mostly only know about WW1 and WW2 US military issued item.  I have some knowledge of Korean War issued items.  I have been making an effort to study the Vite Nam War era items as I come across them.  This set of photos shows a couple Vite Nam War era 1967 dated Radio Receivers that clip onto the US M-1 Helmet.  One is in unissued condition and is missing part of the ear piece but has an extra piece on the wire.  The unissued one also has the 1968 dated shelf tag.  Both Radio Receivers are missing the battery.  I also came across a 1966 dated manual for this type of item.  I show photos from it here.  Hope you people out there on the internet enjoy some free information about these.