Dan a few Omega models of this era (late 1980s to mid 1990s) have no serial on the movement and instead have it printed onto the inner caseback. The reason for this is that the movement is completely replaced when serviced through official channels. The Pre-Bond quartz models are another example but there will be many others too. The issue is unless very careful the original printing can be worn away by over long ultra sonic cleaning for example. As me how I know this...