Chemin de Fer du Nord Railway Seal, Image by StuE, Found by PWS
Found in the Colchester area, 19mm.
Both sides have a thick raised circular border. On one side it appears to have a floral pattern and the number 3 clearly at its centre. The other side has the letter A at its centre and around the border is a star followed by CHEMI---EF?-?-R- joining up to the star again.
From a private communication with Paul Cannon, "The following relates to seal 1 and seal 2.
I believe that both of these are railway seals from the Chemin de Fer du Nord Railway in France. This group of lines linked Paris northwards to the Channel ports and also into Belgium etc. For a history of the railway, including map see Wikipedia.
One of the seals is more easily read than the other but by comparing them with examples on the French website below I suggest that the main inscription reads: 'CHEMIN DE FER DU NORD', around as you say, an 'A'. The other faces would no doubt contain a station name but I cannot make these out because of the corrosion.
For other seals from this railway see PLOMBS DE SCELLES DU XIX ème SIECLE A NOS JOURS > LES TRANSPORTS."