What characteristic differentiates the M coupler from the A coupler?

The characteristic that sets the M coupler apart from the A coupler is indeed related to the probe design. The M coupler utilizes a different probe design that is specifically engineered for connecting to specific types of kegs, often in the context of certain brands or styles of beer, such as those used in the U.S. craft beer market. This design modification enhances the performance, allowing for better beer flow and decreasing the risk of leakage, which can be critical when serving various types of beer under different conditions.

In comparison, while the body design may also vary between couplers, the critical functional aspect that impacts operational efficiency and accuracy when dispensing is primarily dictated by the probe. The material used in the construction of these couplers, though important for durability and sanitation, does not define the primary functional differences that impact their use. Similarly, the coupling mechanism, while it might vary in terms of locking and unlocking methods, does not specifically distinguish these two couplers as effectively as the unique probe designs do.