What is the significance of the letter e on food labels for example 450 g e or 1.0 L e?

In Europe, more than 20 years ago, the law changed from minimum weight declaration on the label to average weight declaration. This means that where before you were guaranteed at least the weight declared on the label in every pack, you now may get more or less in individual packs. The manufacturer has to demonstrate to Trading Standards (Weights and Measures people) that on average over many many packs the average weight is what is declared on the label.This is why there is the "e" to confirm that the weight declared conforms to European Average Weight legislation.