[CTPP] SF1 API variable IDs

So I'm once again befuddled by the way the API handles variable IDs. I'm trying to retrieve data from table PCT20: "Group quarters population by group quarters type" from SF1. However the SF1 does not seem to follow the same variable formatting as the ACS 5-Year (it doesn't seem to use the _001E / _001M suffixes). In the example queries, the table "P001: total population" is retrieved using the ID 'P0010001'. Based on this I would've guessed that the 0001 designates the variable within the table in the same way that _001E does. However using this same style suffix on PCT20 results in an error. Does anyone know how these IDs are constructed? I haven't been able to find a variable list for SF1 that corresponds to this one<http://www.census.gov/developers/data/2010acs5_variables.xml>ml>. Thanks much, Sam Ennis