Which era are you talking about? 50s and 60s there are a lot of casebacks without the logo but with correct markings on the inside. These were the original casebacks unless in some cases the logo was polished out.
Some Seamasters from the mid 50's through the 70's had raised relief hippocampus logos, others had shallow engravings of hippocampus logos, and some had no hippocampus at all. You'd have to let us know what reference number it is and give us pictures for us to let you know if it should or shouldn't have the logo.