Brand new 48V 500 mA center tap AC transformer.
Input is 110V/220V.  Output is 24V + 24V.

It measures W 42mm (67mm with the mounting tabs) D 40mm H 35mm

You get 48V by using the two YELLOW wires only.  Black would not be used.
If you need 18V you can use one YELLOW and one black wire.  Any of the YELLOW can do.

This is an AC transformer and the output is AC also.
If you need DC you need a power supply not a transformer.
If you try to measure the output using the DC setting in your meter it would not give you a voltage.
You need to set it to AC in order to measure the output voltage.

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