It can be numbers and letters or some thematic pictures, depending on slot type.
http://kronmark.eu/index.php/forum/more-about-the-kunena/40121-gry-na-pieniadze-prawdziwe-albo-polskie-legalne-kasyna
https://one-diz.3dn.ru/forum/12-14256-1#20511
https://www.ww2.site/forum/viewtopic.php?t=107769
https://www.ww2.site/forum/viewtopic.php?t=100015
https://neoternia.net/viewtopic.php?p=273563#p273563
http://forum.bandingklub.cz/viewtopic.php?f=36&t=1209119
http://forum3.bandingklub.cz/viewtopic.php?f=38&t=1447244
http://webidsupport.4up.eu/forums/showthread.php?tid=316157
http://horsecity.ru/phpBB3/viewtopic.php?f=8&t=326437
http://www.prepody.ru/topic48659.html
http://mathinference.com/forum.php?mod=viewthread&tid=1452&extra=