Tom Hamilton of Mora, Minn., turned 85 this week, enjoying his favorite restaurant seafood buffet with his wife, Gwen, and a stop at Grand Casino Hinckley.

But the celebration for Hamilton, a cancer survivor, was understandably muted. The recent death of his sister-in-law, Madolyn Mann, has left a hole in his heart — and in his mailbox.

Of all the offbeat subjects I’ve tackled, this story is among my favorites — a lighthearted homage to family bonds, sustained silliness and the thing I love almost as much as I love my kids:

Handwritten letters.

Since 1975, when a postage stamp cost 10 cents, Hamilton and Mann sent the same birthday card back and forth. That’s 40 years and 80 trips of witty tidings penned on shrinking white space, the inevitable rips gently repaired with tape.

“We kept the post office in business,” Hamilton said.

Under the heading “Card History,” throughout the years the family documented the card’s arrival in Texas for Mann’s May 7 birthday and in Minnesota for Hamilton’s June 7 birthday.

“This card looks almost as old as us,” Mann wrote to her brother-in-law when he turned 84 last year. He didn’t know then that the card’s traveling days were over. Mann died in October at 83.

“We don’t realize we’re this old,” Hamilton said. “But I guess we are old.”

Mann and her husband, Arvid, who died in 2012, first sent the card to Hamilton for his 44th birthday in 1975.

“Happy Birthday to a good-looking relative,” the card says. The inside inscription reads: “Save this card. You can send it to me on my birthday.”

A year later, Hamilton did just that. He mailed the card back with a message penned by Gwen, because he has horrible handwriting.

“Bet you didn’t think we’d hang onto this card so long, but it was something we could use for more than a one-shot deal,” Gwen wrote. “Ha ha! Happy Birthday.”

Back and forth it went. The 13th time: June 7, 1981 (“The big 5-0 for you, Tom!”); 20th time: May 7, 1985 (“This card has really held up”); 38th time: May 7, 1994 (“Happy Social Security Day”); 43rd time: June 7, 1996 (“Happy Medicaid Birthday”); 50th time: May 7, 2000 (“A new century, but the same old card”).

Inside every time were the same two $1 bills.

“They were good buddies,” Gwen said of her husband and sister. “Tom was an only, so my sister and brother were his family. We always were together.”

The Hamiltons, married for 65 years, never imagined their little joke would go this far. One year, Gwen thought that she had lost the card “and was just sick about it.” She found it tucked behind some papers.

Gwen and Tom were sweethearts at Minneapolis’ Central High School. He worked for 37 years as a trucker. She worked as a printer and supervisor at Lifetouch. They’re the parents of two children, and have four grandchildren and six great-grandchildren. Now retired, they winter in Florida and Georgia.

Mann did office work for Lutheran Social Service. She and Arvid, a Ford assembly plant supervisor, were matron of honor and best man in the Hamiltons’ wedding and raised two sons in proximity to the Hamiltons. The couples often traveled together.

After retirement, the Manns moved to Pharr, Texas, but they still visited relatives in Minnesota every summer. After Arvid died, Mann moved back to Minnesota.

“She was a real forward woman,” Hamilton said of his sister-in-law. “She had definite opinions on everything.”

Yet, her birthday greetings remained sentimental, championing family love, a playful spirit and continued good health.

The Hamiltons saw Mann at a family reunion shortly before she passed away last year. Her life was never the same after Arvid died, Gwen said. “Living alone after a 63-year marriage was more than she could handle.”

A few weeks ago, Hamilton asked his wife if she might mail him the card for his upcoming birthday, in Mann’s stead.

“I told him absolutely not,” Gwen said. She didn’t think that seemed right.

But she, too, marvels at how much fun it was across so many miles for such a long time.

“Everyone always asked, ‘Did you get the card?’ ” Gwen said.

“You didn’t have to say, ‘What card?’ ”