South American M1 paratrooper helmet

This is one of the helmets I acquired some time back in a lot from South America, its a paratrooper configuration. The shell is a surplus rear seam  stainless steel rim band surplus helmet given to south america. the liner is a bit enigmatic as I can't find information on it. it looks to be a US made liner (i think) and has the markings of 3 circles, inside the left circle is ISW, the center circle is blank, the right circle has 68.  I think this is either a Vietnam early era helmet IE: the 68 or 68 is the size, I could not find a maker identification regarding the ISW . Some corrosion on the metal clasps and such. Please see the photos or ask any and all questions prior to bidding as this is an as is no return item. Ebays system is currently full of bugs since they went to the new seller format, so please be advised we only ship UPS Their system however keeps changing things to offer usps which we do not use, we do not ship to PO boxes, we do not ship international . The system is failing to notify buyers and circumventing the preferences we set for selling so please do not bid if you have a PO box or are out of the country or have an issue with UPS deliveries. Again we only ship via UPS!!
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Most these helmets I have seen and have had have spanish made liners but I am guessing this is a surplus US liner given to south america and modified.but I really don't know. It is an interesting variation.