This is the Scalextric Transformer 15V DC 4A Output with 6.5mm Round Plug for Digital Sets. This power supply can also be used as a second power supply with digital power bases to allow for running up to six cars.
If your Scalextric analog power base has one wide flat jack in the center of the power base then it is compatible with the P9403 transformer.
If your Scalextric analog power base has round power jacks then you will need to use transformer P9203.
For Scalextric Digital use the P9303 transformer (this one).
Used with permission, copyright 2019 Hornby (Scalextric).  Copyright 2019 Brey Corp. t/a Hobby Works.