The variety formerly known as the ruby eyed mocha, havana has a very similar story to the coffee variety – it’s a brown self rat that is a combination of fairly common genes, grandfathered in from the old Unstandardised classes, and it involves a bit of luck and selective breeding to get the correct shade rather than just an odd mink colour, even once you have the correct genotype. In my experience, some rats which seem the correct colour (or at least have the ruby eyes as kittens rather than black eyes) will darken as they mature into minks rather than havanas.
Some lines of American mink have ruby eyes and a different coat colour to UK minks. These can show well as havanas.
Genetics: Mink with one copy of the red eye dilute gene (aammRr). Although red eye dilute is technically a recessive, in certain circumstances it will affect colour even when only one copy is present – and on a mink background is one of those times. Alternatively, aaamam (American mink) from certain lines.
NFRS standard: To be a warm rich brown. Eye colour mid ruby, to harmonise with coat colour.