The Dragons lock played a pivotal part in the campaign overall, starting the defeat to Ireland and the victories over France and Italy before reverting to the replacements' ranks following the return of Alun Wyn Jones.
The 28-year-old was one of the stars of those early games in particular, proving he belongs at the top table despite having led a life as a computer software salesman and a semi-professional player only three years ago.
"I can't quite believe I've got that medal. It's amazing and I'm so happy with what the team have achieved," said a modest Coombs.
"I didn't play much of a part in those last two weeks, apart from a couple of minutes against England, but it was fantastic.
"It's so special. It's my first team medal in senior rugby and it's quite a good one to get!
"I could never have expected this at the beginning of the season but, as the year went on, after I got back from injury, I was playing some decent stuff.
"With all the injuries to second rows I was watching it closely and I'm happy with the way it went.
"You just can't put it into words. It was so phenomenal just to be part of. I watched them win the Grand Slam last year and you can't picture yourself in that set-up - it's amazing."