Way Back When...
You've certainly seen the birthday cards with dates on them stating, "If you were born on this date..." and then it proceeds to list a few historical significances for the month and year of your birth. Do the same thing, using the 50th wedding anniversary date. Do an Internet research typing in the anniversary date as well as the words "historical significance," and you'll likely discover some interesting facts about the world when the couple was just starting out. Use a few of these facts to highlight the invitation, ending with the fact, "John and Ann Lane were married." In addition to these fun facts and the premise of your invitation, you'll then need only to add the date, time and location of your party, as well as a request for everyone's RSVP.
No, this doesn't mean run out and purchase the latest addition of the computer software PhotoShop. Instead, find some unique photos of the 50th anniversary couple from way back when and see if they need a bit of restoration from an actual photo shop. Once the photo or photos are restored, incorporate one or more into the anniversary party invitation. There are numerous online sites that feature templates for making your own greeting cards or invitations. Visit Snapfish, Kodak Gallery, Vista Print or Shutterfly and choose your template. Upload the photo you've stored by scanning it to your computer, then simply fill in the blanks as needed. You're invitations will arrive custom made in no time.
If it's possible, try to locate an original invitation from the anniversary couple's wedding. Take it to a print shop that specializes in elegant invitations and ask them to recreate a 50th anniversary party invitation as close to the original as possible.
Another option involves making photo copies of these original wedding invitations and including a copy inside every invitation that is sent for the 50th anniversary.