Final answer:
Some fire extinguishers do not require a number before the classification letter, such as dry-cleaning solvent and foaming plastics extinguishers.
Step-by-step explanation:
In fire extinguishers, there are certain classifications that indicate the type of fire they can extinguish. These classifications are represented by letter symbols, such as Class A, B, C, D, and K. However, there are some fire extinguishers that do not require a number before the classification letter. Examples of these include dry-cleaning solvent and foaming plastics fire extinguishers.
While they were once recommended for use, dry-cleaning solvent fire extinguishers are no longer recommended due to their potential health hazards. Similarly, fire extinguisher systems for foaming plastics fires do not require a number before the classification letter.