Both types have a 12mm extension on the bottom of the acroterion which serves as a means of attaching the ornament to the gutter.
On the top-attaching version this extension is bent at a 90° angle (i.e. parallel to the ground). This allows the acroterion to “sit” on the top edge of the gutter.
On the inside-attaching version this extension remains pointing straight down (i.e. perpendicular to the ground, and parallel to the side of the gutter). This allows the acroterion to be attached to the inside of the gutter.
The elevations of uninstalled straight-bottomed (inside-attaching) acroteria are in each case nominally 12mm higher than those of their bent-bottomed (top-attaching) counterparts to the intent that the visible part of each type above the gutter line should be the same once installed. This is true as long as the bottoms of inside-attaching acroteria are fixed no lower than 12mm below the gutter line and is subject to the effect of parallax when viewed from the ground.
Top-attaching centre acroteria have a decorative swirl or scroll below that part of the “prow” that projects horizontally across the front of the gutter line, but the smaller projection of the prow of inside-attaching acroteria precludes this decoration.