A B757 was dispatched into the Western Atlantic (WATRS) route structure with a single FMC. A MEL applicable to this aircraft which was purchased recently was incorrect because the applicable switches were not installed.

Narrative

After a last-minute aircraft switch; the right FMC failed during preflight. Maintenance applied MEL 346101 Condition C for the failure. I noted that the operations procedure for Condition C involved use of a rotary switch for alternate FMC selection not installed on my aircraft (a push button switch was installed) and communicated my concern to the controlling Dispatcher who contacted Maintenance Control. The Dispatcher and I were told that the MEL was appropriate for long range navigation and to use the installed switch. Our flight was operating through the Western Atlantic Route System (WATRS) to the Caribbean. While enroute; I still felt uncomfortable with the MEL and referred to both the MEL and the company FOM. The FOM stated that single FMC operation into the Western Atlantic Route System area was prohibited. There was an exception for 3 recently purchased newer aircraft. These have the aforementioned rotary switch installed. When within ACARS range; I contacted the controlling Dispatcher and informed him that I felt the applied MEL was inappropriate; but that MEL 346101 Condition A was. I also requested that the return flight be filed outside the Western Atlantic Route System area within 162 NM of land. He shared my concern. After landing; a 3-way telephone conversation took place between the Dispatcher; Maintenance Control; and myself. The appropriate MEL (346101 Condition A) was applied and we returned to the United States outside of the Western Atlantic Route System area. In my previous experience; any MEL involving an airspace operating restriction stated said restriction in the MEL application procedure itself. In this case; this was not done; let alone any reference to any potential FOM restriction. I believe that the latest MEL revision simply used the former language regarding FMC MEL procedures. While I am not sure; I believe that the former 757s have the rotary switch installed (I've never seen one except at a distance). However; approximately 30 years later; B757s have the push button switch that is totally ignored in the MEL. This is a situation ripe for confusion; let alone a potential FAA violation. I feel strongly that this particular portion of the MEL should have never been approved as it does not address aircraft differences within the fleet; and am curious as to what other similar inconsistencies that might exist in the document.