What are the dimensions of the replacement seat?
The dimensions are 12” x 14.5”
where do I find the serial number?
The serial number can be found on the box it came in or somewhere on one of the metal bars that support it.
How does that black cord attached to the frame under the seat work? I know it starts the folding process, but how is it wrapped after folding?
It is a solid bar that initiates the folding and doesn't need to be and can't be wrapped.
Will this height adjustment knob fit my Carex Rollator walker model#A221001702002379CB ?
Yes, this will fit your rollator. Thanks for reaching out!