How to get the real, internal Genre value (number)?

As far as I know the Genre is internally encode as a simple number in a MP3 header.

MP3tag shows in the column "Genre" the corresponding Genre as full label text.

E.g. 145--->Pop

It works in Mp3tag. However in (other) audio players (on Android) I got some strange values as Genre. So would like to peek directly into header to get the real internal Genre value for further investigations.

How can I tell Mp3tag to show me this number (and not the interpretation)?

Are there differences between ID3v2.3 and ID3v1 Genre encodings?

MP3tag has no option to show the used number.
The number is only used for ID3V1 tags.