The Ronda 1217-21 watch movement features a bimetallic screw balance and a base plate that has been shortened compared to the Felsa 55 movement. The base plate of the Ronda 1217-21 movement is equipped with four diagonally placed rests. Certain registers mention the use of similar parts for both the Ronda 1217-21 and Felsa 55 movements, hence the data for the Felsa 55 movement is mentioned in brackets. The Ronda 1217-21 movement is a manually-wound movement that was commonly used in the period around 1930.