How to do this effect: a type of "alpha" mask

Hello,

I have to recreated this effect from a component. The component resides in a Flash 8 FLA, but it is published for Flash 6 and uses AS 1.0 OOP programming.

My goal is to re-create the transition shown in the image below. As you can see it appears to be a type of “alpha” mask, but obviously Flash 6 or 7 doesn’t support that feature.

Does anyone have any idea how this was faked?