When is a labeling field the deciding feature?

The 5UB1923 takes the base 5UB1921 and adds exactly one feature: a transparent labeling window on the cover plate. This makes it the right choice when circuit identification is required by building code or client specification, but the physical safety add-ons — hinged cover and increased touch protection — are not needed. Office fit-outs and commercial projects are the textbook scenario: dozens of identical carbon metallic sockets in a single open-plan space, each needing a circuit number visible to maintenance staff, and the environment is adult-only so the flap and touch protection add cost without adding value.

How does the 5UB1923 differ from the base 5UB1921?

Electrically and mechanically the two are identical: same 10/16 A rating, same screwless push-in terminals, same carbon metallic finish, same 55x55 mm plate dimensions. The sole difference is the labeling window — accounting for the 14 g weight delta (92 g vs. 78 g) and a roughly 10–15 % unit cost increase. If your project specification does not mention circuit labeling, the 5UB1921 is the same socket at a lower cost. If labeling is required, the 5UB1923 delivers it without paying for features you do not need.