IPPROTO 33 is DCCP in the IANA Registry:
https://www.iana.org/assignments/protocol-numbers/protocol-numbers.xhtml
IPPROTO_SEP was added about 20 years ago in r33804. The entries were added
straight from RFC1700, without regard to whether they were used.
The reference in RFC1700 for SEP is '[JC120] <mystery contact>', this is an
indication that the protocol number was probably in use in a private network.
As RFC1700 is no longer the authoritative list of internet numbers and that
IANA assinged 33 to DCCP in RFC4340, change the header to the actual
authoritative source.