The usual practice in the 18th century was to have the movement between two parallel plates and the balance wheel outside the top plate.

These actuators are mounted in pairs to the mechanism's base, crossing over to three mounting points on a top plate.

Some time after that the engraving of the top plate was dropped and instead an engraved disc was placed under the shutter speed dial.

Rather than merely a slightly-curved top plate with triangular gussets, it was now possible to roll a semi-circular plate.

Another method was to make a much deeper mortise in the top plate.