Where does the Geonames admin_code1 for Russia come from?

Looking at the data dump of post codes for Russia (country downloads | Russian download link) I can’t understand what the admin_code1 corresponds to.

Looking at the ISO3166 sub regions, I see the codes are characters (whereas in geonames the codes are numeric). Looking at Wikipedia I can’t find a numeric reference for regions anywhere. I was tempted to order alphabetically and take the row number, but some numbers are missing (2 for example).

In the readme.txt geonames lists admin_code1 as: 1. order subdivision (state) varchar(20), but this is just generic to all countries and gives no source. I’ve also checked the Russia page, which links to the St. Petersburg’s administrative website, but there’s seemingly nothing useful here.

What do the numbers here represent? I need to link the ISO 3166 subdivisions to the countries in geonames, thus I need to create a lookup, is there an existing resource, or way to do this?