Some of us have backed "Starship Class" as the proper class designation for TOS Enterprise in a previous thread for the reason you stated. Many however, have preferred the use of other class designations due to their appearance in on screen episode schematics (which are assumed to be of Enterprise), fan produced material, or other sources. I'll go with what I've seen on screen, as opposed to what somebody wrote down on a piece of paper saying that is what they think it is. Every excruciating detail wasn't thought out in advance like today's TV audience expects. The show was produced to the best of the ability of cast, crew and staff as time and money allowed. Waiting to be rebutted into oblivion.