How should I set the source_srs parameter of LayerMapping() in GeoDjango for the 2000 Census data?

I've downloaded 2000 Census zipcode polygon data as a .shp file from http://www.census.gov/geo/www/cob/z52000.html#shp

According to the GeoDjango docs, I need "an integer SRID, WKT or PROJ.4 string" for my data set.

http://docs.djangoproject.com/en/dev/ref/contrib/gis/layermapping/#layermapping-api

I googled for the relevant parameter and all I could find was the following message, which mentioned 4269 and 2163. Neither one worked. http://old.nabble.com/RE%3A-SRID-and-unprojected-%28Census%29-data-p15996981.html

Note: when I opened the .shp file as DataSource and tried to see the srs attribute of the layer, here's what I got开发者_StackOverflow:

>>> srs = lyr.srs
>>> print srs
None

So, what value should I use for source_srs?

Most probably the data comes in WGS84. The SRID is 4326.