RexP V1
You will need this adapter for the V.1 Rex P. The RexP V.1 has holes on the shroud.
https://donnyfl.com/products/rex-p-m18x1-adapter-a68?_pos=1&_sid=11ad22a71&_ss=r
I highly recommend the Emperor .45/.50 M18x1 #S5B for this airgun.
https://donnyfl.com/collections/ldc/products/2-x-10-5-inches-emperor-v3?variant=14050945105963
New REX-P V2
This adapter fits the newer V.2 REX-P. The Newer Rex p has lines on the shroud.
https://donnyfl.com/products/gamo-tc45-m18x1-adapter-a59?_pos=2&_sid=c88d29ff6&_ss=r
I highly recommend the Emperor .45/.50 M18x1 #S5B for this airgun.
https://donnyfl.com/collections/ldc/products/2-x-10-5-inches-emperor-v3?variant=14050945105963