Markings on Ladies Watch Case

by Norman
(Suffolk , England)

The marks on the inside of a ladies watch case are :-

18k in a rectangular box.
An imprint of the Queens head
AB in a box
A series of numbers :- 15 150 71
I`m new to Gold and its Hallmarks and find the variety of the subject fascinating. My watch appears to have some but not all of the marks described as "compulsory".
I'd be grateful for any guidance you can provide.
Regards
Norman