How to classify Census work on Tax forms?

I am filling out my tax forms and since I worked for the US Census last year I have to write down the "income" I made from doing that work.

One of the forms that I am filling out is a Schedule C.

The Schedule C has  "Box B" in which I am supposed to write in a 6-digit code that identifies the kind of work  I did. 

There is a list of codes in the instructions ("Principal Business of Professional Activity Codes")  according  to whether the work was  related to , let's say, agriculture, or food services, or  construction of buildings, or  educational services, or manufacturing, etc. .

However  what I learned from working for the Census is that it is part of the Department of Commerce. I wonder why there isn't a separate and "special" code that would strictly identify Census work since it is  not exactly under the umbrella  codes of  "Administrative" or "Professional, Scientific and Technical Services"  or "Remediation Services"  etc.

It does seem that the best available code  might be "561600" which is for "Investigation and Security Services."  (Census workers who go door to door asking for information are like "investigators." 

Or maybe code 561110  for "Office Administrative services"?

However Census enumerators do not work in an office. They have to be going to different buildings and dwelling units.

You would have to be familiar enough with Tax forms and their different criteria to know what I am referring to, but  what is the best code to identify Census-enumerator work?