A CAS Registry Number identifies one specific substance, whatever it is called. Different forms of the same peptide, such as the free base and a salt, or a linear and a cyclised form, can carry different numbers, so the number should match the form actually supplied.
Some research peptides, particularly newer or less studied ones, have no CAS number assigned.
See also
For in vitro laboratory research only. Not for human or veterinary use. This library describes chemistry, analytical methods and published laboratory research; it gives no guidance on use in people or animals.