First, for a 403, the number given above the gazoline pump is the serial number of the car.
You should have the same on the metal plate on the car.
You can find the engine number just above the ignitor, on the engine. It begin with a letter.
From A to K, it should be a 8CV, after, a 7CV.
I don't recognize the serial number you give.
It should be a 7 numbers serial, beginning by "2" for a 8CV, or "3" for a 7CV.
Yours is only a 6 number serial, and if it begins with a "4", this engine could have been taken on a specific industrial equipement fitted with peugeot engine. forklift or fire waterpump ?
But I think that the number you give haven't been taken neither above the gazoline pump nor the ignitor.
Find these two number and we'll check.
I hope this answer is ok,I haven't any document in here so it's my memory.
I wish specialist will confirm or precize these elements.
504 C12 1971 en cours de redémarrage