Python CGI with IIS6 returns a 404 error

I'm trying to get Python 2.7 working as a CGI script in IIS6. I do this all the time on W2k8, but this is the first time I've tried to use it on IIS6 and I cannot get it to work. Googling suggests just two steps are needed:

1. Create the Web Service Extension
2. Add a mapping for .py

I've done both of these but when I try to open a test.py page I get a 404 error. There must be something simple I'm missing. Any suggestions gratefully received while I still have some hair left!

I'm testing on a new install of W2k3R2 so there is nothing extra like URLScan or AV that might be interfering.

I created the Web Service Extension using:

I'm using Python in a virtual directory, and the directory properties are:

And finally the mapping is: