Factual error: While the show always made it winter time by having snow on everything (salt piles strewn about) and icicles on all the windows, this episode has an actual date of occurrence, June 6, 1944. They help to solve the snow on the set by taping the whole episode inside. However, the windows still all have the ice formations on them. It's late spring.
Factual error: The motorcycle courier coming in wears sunglasses that are definitely newer than 1942. Sunglasses with domed, wrap-around lenses were not invented in the 1940s.
Answer: It's a comedy, not a documentary.
stiiggy
Perhaps it was counterfeit. There are numerous episodes where they deal in counterfeit monies.