In the transaction trace the GUID is called CorProcessData. The http header that is transmitted over the wire is called CorID and included a lot more, e.g. the sequence number.
Example:
GET /MathProxy/rest/hello/mathcomplex?operation=mean&values=1,2,3,4,5,6 HTTP/1.1
CorID: B63D31070A82B2D560A6C783CD3209CA,1:1,0,0,,,AgAABClIQgAAAAFGAAAAAQAAABFqYXZhLnV0aWwuSGFzaE1hcAAAAAlIQgAAAAJGAAAAAgAAABBqYXZhLmxhbmcuU3RyaW5nABFBcHBNYXBDYWxsZXJBZ2VudEhCAAAAA0UAAAACAApNYXRoQ2xpZW50SEIAAAAERQAAAAIAEEFwcE1hcENhbGxlclR5cGVIQgAAAAVFAAAAAgAKV2ViU2VydmljZUhCAAAABkUAAAACAA9DYWxsZXJUaW1lc3RhbXBIQgAAAAdFAAAAAgANMTQ3NjIzMTI0NjA4N0hCAAAACEUAAAACABZBcHBNYXBDYWxsZXJNZXRob2ROYW1lSEIAAAAJRQAAAAIAI1dlYlNlcnZpY2UgYXQgaHR0cF8vLzEyNy4wLjAuMV84MDEwSEIAAAAKRQAAAAIAEEFwcE1hcENhbGxlckhvc3RIQgAAAAtFAAAAAgANYXBtLWltbWVyc2lvbkhCAAAADEUAAAACABFVcHN0cmVhbUdVSURDYWNoZUhCAAAADUYAAAADAAAAE2phdmEudXRpbC5BcnJheUxpc3QAAAAKSEIAAAAORQAAAAIAIEI2M0QyN0ZDMEE4MkIyRDU2MEE2Qzc4Mzc2RERDRkQzSEIAAAAPRQAAAAIAIEI2M0QyODhDMEE4MkIyRDU2MEE2Qzc4MzdGRjc0RTk3SEIAAAAQRQAAAAIAIEI2M0QyNzk4MEE4MkIyRDU2MEE2Qzc4MzhCMDVDNDdESEIAAAARRQAAAAIAIEI2M0QyNTkwMEE4MkIyRDU2MEE2Qzc4MzU1Qzg5RkRBSEIAAAASRQAAAAIAIEI2M0QyODBGMEE4MkIyRDU2MEE2Qzc4M0NBNTM3ODRBSEIAAAATRQAAAAIAIEI2M0QyNTk1MEE4MkIyRDU2MEE2Qzc4MzRCRUNBQUM5SEIAAAAURQAAAAIAIEI2M0QyNDJCMEE4MkIyRDU2MEE2Qzc4M0QwMzA3M0JDSEIAAAAVRQAAAAIAIEI2M0QyNzVDMEE4MkIyRDU2MEE2Qzc4MzMyQUU4RTM0SEIAAAAWRQAAAAIAIEI2M0NEREY3MEE4MkIyRDU2MEE2Qzc4MzkyQjQyRUM5SEIAAAAXRQAAAAIAIEI2M0NEQjI3MEE4MkIyRDU2MEE2Qzc4MzcxOTNCMDNGSEIAAAAYRQAAAAIAE0FwcE1hcENhbGxlclByb2Nlc3NIQgAAABlFAAAAAgAGVG9tY2F0SEIAAAAaRQAAAAIAClR4blRyYWNlSWRIQgAAABtFAAAAAgAkQjVDODgyREEwQTgyQjJENTYwQTZDNzgzOTY3QzNENTA0MjI5SEIAAAAcRQAAAAIADkFwcE1hcEFwcE5hbWVzSEIAAAAdRQAAAAMAAAACSEIAAAAeRQAAAAIACk1hdGhDbGllbnRIQAAAAB4=
Host: 127.0.0.1:8010
Connection: Keep-Alive
User-Agent: Apache-HttpClient/4.5 (Java/1.7.0_101)
If I'm not mistaken "B63D31070A82B2D560A6C783CD3209CA" is the CorProcessData, 1;1 the sequence number.
Ciao,
Guenter