Factual error: When Sam reads the newspaper, one article claims that 254 people were killed in a Paris air raid. This episode takes place in February 1941, when Paris was already occupied by the Germans, and England did not bomb Paris. Oddly enough, this same article appears in a previous episode.
Enemy Fire - S3-E2
Factual error: While repairing Mary Wrenn's lights Gordon Drake says "Someone fused the ring main with a 3 amp fuse." It was February 1941 but ring mains were not introduced until 1947. (00:14:50)
Factual error: A sign says "REMEMBER THE ENEMY IS LISTENING" in Chicago typeface, but this font wasn't invented until 1984. (01:15:50)
Answer: They are using the accepted names of the bombs. During World War 2 the Germans used the metric system and produced bombs ranging from 50kg to 2500kg. Referring to the weight of the bombs in pounds (or fractions of tons) risked confusing the situation and would require someone to understand how to convert from imperial measures to metric, not an easy thing to do when under stress.