Get Omega Speedmaster Mark 4.5 US. An interesting read on the omega caliber 1045 can be found here, as documented by the legendary chuck maddox. All with the same movement, but with different case shapes.
41.5 mm without crown and 44.5 mm with crown.
These mark 4.5 dials (actually there are a few types of them, original or service) can be identified immediately by: Omega speedmaster mark 4.5 vintage swiss automatic chronograph watch ref 176.0012 cal omega 1045 (lemania 5100). I have a mark iii and a mark iv, ages ago i had a 4.5. Black omega chronograph dial, applied white baton hour markers, white minute and hour hands, central chronograph second hand, subdial second, 12hr totalizer and 24hr indicator, day date at the 3, tachymetre outer dial, hands and dial in excellent condition.