MSNWatch
··Vintage Omega AficionadoHere is another soure of reference, but it only pertains to the 2998's. I'd like to hear from Mike or others if there are any errors that they see. This is a translated page but it has a lot of info.
http://translate.google.com/translate?hl=en&sl=da&u=http://www.vintageure.dk/DesktopDefault.aspx?tabid=326&prev=/search?q=vintageure.dk+omega+speedmaster+ref.+298&hl=en&qscrl=1&rlz=1T4TSNO_enUS523US523&biw=1619&bih=760&sa=X&ei=Bq83UdKmPKPFyAGbrYG4Bw&sqi=2&ved=0CDIQ7gEwAA
Most informative and pretty accurate from what I can tell - the author's experience down to the serial numbers he cites for the various 2998 references coincides with my observations.