The system of two-position signalling is the older system on the Belgian Railways. Many lines, primarily the less important ones of the network, have retained this system until it was replaced by light signalling, or until closure of the line.
When the three-position system was introduced in 1919 together with a new style of semaphore arm, a 40 cm diameter round disk plate was added to the semaphore arms that would be used in the older system. This method of distinguishing the two systems was continued when the final style of semaphore was introduced in 1931, the model with the 'trinocle' of coloured glasses incorporated.
Information about the proper application of two-position signals can be found in Signal Rule books of the Belgian Railways. Many documents (most of them in French, some in Dutch) have been collected on this invaluable website: