I'm batting for a 19mm lug width, omega seems to use odd lug sizes. Also, keep in mind the SM300 has 21mm lugs and a 41mm case diameter. It also uses a twin barrel movement while the Railmaster above clearly has a single barrel 8806. Therefore I think we can assume that the smaller movement was used in order to reduce case size. If it was going to be the same size as the SM300 then why not use the double barrel movement? Although, if they've gone for a soft iron inner case or similar then that could also explain it. We will see!
I wait with bated breath!