Those with (original) rare or unusual markings or some kind of documented history tend to be more expensive.
Dating wwii helmets is haden christensen dating
Sometimes US infantry has netting, sometimes they don't.
When Canadian Forces (Princess Patricias Canadian Light Infantry) first deployed to Afghanistan they made use of nets and "scrimmage" (I think that is the correct term..of cloth) on their helmets.
It should be mentioned that Italian researches have found their sperm numbers going up in the last decade - but then Italy loves to be different.
Nobody’s quite sure what’s behind the general drop-off. The trouble with science is not just that there often isn’t a single answer.
Quite why you need so many sperm to fertilize one egg remains a mystery, but a low sperm count or poor sperm quality is the main cause of infertility in about 20pc of couples with fertility problems in the UK, and is an ‘additional factor’ in a further 25pc of cases. The much publicised decline of man has been on the cards since 1992, when seminal Danish research found that the number of sperm in each millilitre of semen has halved since the World War Two, while abnormal sperm (double-headers, big-headers, double-tailers and slow-coaches) is on the rise.