Others here may know better but my suggestion would be to go to a visitor centre, or a staffed Underground/Liz station and check it on a ticket machine. If staff assistance is required to reveal the number you can ask someone for help.
Note – If it doesn’t have a capital ‘D’ in the bottom-left corner of the reverse side, it’s a first-gen card and won’t be compatible with TfL Go.
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This reply was modified 2 months ago by
Edvid.