Check out this link. I asked the same question about a year ago. Some very knowledgable "Lincoln Guys" put in their opinions, along with Mike Diamond. Hope it helps! http://www.lincolncentforum.com/for...e-raised-area-Die-dent-maybe&highlight=1973-D
I found some articles as well and no one is sure what caused it but it's not that rare. It turns out there are lots of these