In 2002, this letter was made public by the addressee. In it, Heinlein says that Lady Vivamus is actually just his Navy dress sword. This does explain why he lavished so much description on the sword in the novel. Heinlein was a brilliant writer and one thing he did not do was waste many words describing hardware. In novels like "Citizen of the Galaxy" and "Rocket Ship Galileo" where space ships are central to the story we get not a word of physical description of those ships beyond a vague reference to their size. But in "Glory Road" we get a long, detailed description of a sword.
It is also obvious that Albion designed their version of Lady Vivamus without this letter for reference. Their sword is a work of art but it is also a large and heavy weapon compared to a Navy dress sword. They took Heinlein's description of the sword as a "meat-axe" and bone chopper seriously.