The flags to select a specific patch don't make
sense for show contents.
This is the relevant part of the help output:
Usage: darcs show contents [OPTION]... [FILE]...
Outputs a specific version of a file.
--match=PATTERN select a single patch matching PATTERN
-p REGEXP --patch=REGEXP select a single patch matching REGEXP
-h HASH --hash=HASH select a single patch with HASH
-t REGEXP --tag=REGEXP select tag matching REGEXP
-n N --index=N select one patch
The only option that is useful here is --tag, because only a tag gives
you a "specific version" of a file.
A good way to fix this would be to replace the nonsense options with
semantically correct ones: --to-match or --to-patch (there is no
-to-hash currently, but --to-match 'hash HASH' would do the trick).
These forms explicitly refer to the set of patches up to (including) a
given patch, where "up to" means "in the order in which they are
currently stored in the repository".