After hearing Bertie Wooster misquote it as "the rank is but the penny stamp" and be corrected by Jeeves "the guinea stamp" about a thousand times I finally looked into it.
It's from the Robert Burns poem "A Man's A Man For A' That," verse 1.
That hings his head, an' a' that;
The coward slave - we pass him by,
We dare be poor for a' that!
For a' that, an' a' that,
Our toils obscure an' a' that,
The rank is but the guinea's stamp,
The man's the gowd for a' that.
And I was left knowing the source, but not the meaning.
AllPoetry.com helped out:
The rank is but the guinea's stamp" means that a person cannot be given a price. The man's character is the true gold.
So a person is overlaid with rank, like a guinea coin is stamped with an image.
Look at me learning things from PG Wodehouse, via Jeeves & Wooster as portrayed by Fry and Laurie.