This is an authentic Longines Deployment watch clasp (REF L639077045). The Longines logo has worn off the top but there is still a logo on the underside. This will fit a deployment clasp that is 18mm wide at the deployment end.
I have given you photos of your Longines clasp from every angle. Please use them to add to this description.
If you are not sure if this will fit your watch or if you have any other questions then message me. I'm here to help you.