How to get binaries if they are available

[Barrie Stott]

I did an upgrade recently and a binary was installed. In the console log were a couple of lines: 

--->  Attempting to fetch bash-completion-2.1_5.darwin_10.noarch.tbz2 from http://mse.uk.packages.macports.org/sites/packages.macports.org/bash-completion
--->  Attempting to fetch bash-completion-2.1_5.darwin_10.noarch.tbz2.rmd160 from http://mse.uk.packages.macports.org/sites/packages.macports.org/bash-completion

As I understand it, the second one is the binary because of the rmd160 part but it seems to be trying to get a non-binary first and only trying to get a binary when that fails.

Is this because only one of binary/non-binary ever exists at a packages site?
If not, how can I get a source package only when there is no binary?

Barrie.