Hi,
It depends on what you are trying to achieve. Keep in mind that you can not download the actual CAD document through the REST API:

You CAN navigate the CAD assembly structure and apply different ConfigSpec (including 'As-Stored') filters. I haven't tried it, but, in theory, it should return the actually used iteration at the time of check-in.
It seems that you can not get a non-latest iteration in the initial queries (when you look up a document by number, for example: /Windchill/servlet/odata/CADDocumentMgmt/CADDocuments?$filter=Number%20eq%20%27TestNumber.prt%27), but there might be workarounds. You may try accessing them (non-latest iterations) by OR directly, or look for them using the SavedSearch odata domain.
Kind regards,
Dmitry.