Vintage Omega information and value

Good morning , mew member, David from the UK. Just looking for some information and value on late fathers omega seamaster 120 ( I believe the reference may be 136.027. The images attached were taken by a friend who is a professional watch photographer and I can get more photos when home. No papers or box. Have the original steel strap. Watch has been serviced and has new glass but all inner workings and bezel are original with beautiful patina and approx 1968 I believe.