If the slide is highly polished blue, it is a Mark IV. If it is a matte blue it is a Mark I.

The Mark IV specification in the Detonics flyer called for a "Combat Selector" rear sight, which had two set screws and replaceable leaves to change the height of the sight. Some of the Mark IVs have shown up with the regular fixed sights -- not sure if they were shipped that way or if a replacement was made. The Mark IV usually said "Mark IV" on the left side of the slide, below the "Detonics .45" engraving. However, I have seen some Mark IVs without the latter engraving.

The C20000 serial series is a carbon steel gun made from the later casting tool. The trigger is a replacement, and it might have been added by the owner or during a repair. I believe the trigger you describe (Videcki style) was sometimes added during a factory repair.

The Blue books have not been very accurate or consistent in describing the model features or in listing values. But like all the used guns, condition is a major factor in setting a value.