Have a little Pearsons book of the Royal Enfield, which has a large chapter on those bikes.
When I locate it, will see if it sheds light on this subject.
Being a 2 stroke, a perfect airtight seal is required of the cases, so they are likely to be well shellaced together, may take some separating. Are you certain all the retaining screws are out ? A rubber mallet is sometimes useful to shock such things loose.