When I was in the Marine Corps Reserve (early ‘80s) I served with a sergeant who was a Vietnam veteran. He was putting in his 20 and his pension was to be based on the highest rank he held. While in Vietnam all the officers in his unit were killed and so he was “field promoted” to lieutenant, a rank he held for only a brief time. Apparently, his pension was to be based on that rank.
Based on Mr. Rogers posts, it sounds like the pension payout scenario might have changed since then.