Hi Sai,
Yes your observation is correct. The external_key field of GRFNMWRTINST table is the Concatenation of ACCREQ/ and Req ID from GRACREQ.
Also the external_key_dis field of GRFNMWRTINST is the REQNO of GRACREQ.
Please feel free to get back to me in case of more queries.
Regards,
Shweta