Bard classes are not conventional object-oriented classes; they aren't even CLOS-style classes. A Bard class is a set of representations that participate in a given protocol. That being the case, a hypothetical Bard cons cell representation could be exactly the same as a TXR Lisp cons cell, or the same as a Common Lisp cons cell. In either case it need not be the only representation of a cons cell in the language.
(I feel like someone is going to object that I shouldn't use the word "class" for a concept that is so different from what it usually means in object-oriented languages, and that might be true. If someone suggests a better term for a set of representations defined by a protocol in which they all participate, I'll consider adopting it.)