Metal roof I just looked at was nailed between the ridges. I’ve always understood you nail through the ridge. What is acceptable?
I have never seen a metal roof nailed on the ridges always between them around here plus we don’t use nails, screws with a rubber seal
Charlie,
I’ve seen what you’re talking about, but I think it depends on the manufacturer.
This doesn’t make it right, but…click here.
Pro panel brand requires fastening as shown. Besides, those look like screws.
http://abcmetalroofing.com/images/trim/detail_fastenerLocation.png
Here is the fastening detail from ABC Metal roofing, I think this is common throughout the industry.
This is from their website technical bulletin. I know years ago you could buy nails or screws but nails are no longer recommended except for open purlin roofs, not sure why.
Technical BULLETINS
**FASTENER INFORMATION **
American Building Components recommends a #14 X 1”, Type “A”, hex head fastener with washer for all thru-fastener panels applied over a plywood or OSB substrate. The use of #9 or #10 “wood grip” type fastener into a plywood or OSB substrates is not recommended.
**PLEASE NOTE **
This bulletin refers to thru-fastener panels installed over solid decks only. Open purlin construction, such as 2 x 4’s on 24” center, should be fastened with #9 or #10 “wood grip” type fasteners.
Here’s a link to their web site, lots of great info for metal roof installs.
As you will notice with the details of this Manufacturer, Nails are on the rib and screws are on the lower flat sheet.
Many moons ago when I installed Corrugated Alluminum roofing panels on the roof and walls of Chicken Barns and Potatoe sheds, all we had was nails and was always nailed on the top of the corrugation.
Must have something to do with sealing, the nails loosen up after the wood shrinks. 
Even though I am not as old as you, this is my experience also. As soon as you had the first season and a good wind you would be repairing it the next year.
Its not the age its the milage in your pic you look a lot older:p