rsync and ssh port forwarding (2nd try)

I don't think my first posting on this was very clear. Let me try again.

I am trying to `rsync -e ssh' with a work machine "B" from my laptop
at home.  I only have direct access from home to work machine "A".

For ordinary use of `ssh' and `scp' I use port forwarding via "A" to
"B" from my laptop when I use it from home.  Thus, in the background
on my laptop (when I'm home), I have running:
`ssh -N A.WorkDomain  -L  N:B:22'

where N is the localhost (laptop) port number that I want to forward,
and "A.WorkDomain" is the fully qualified name of my work machine "A".
This allows me to do things like:
`ssh localhost -p N'  and  `scp -P N localhost:/tmp/foo /tmp'

which then connects "directly" to "B" at work (via "A").

But now I need to rsync a directory on B to my laptop at home using
`rsync -e ssh'.  

Were my laptop at work,  I'd just do (say):
`rysnc -a -e ssh --delete B:/tmp/ /tmp/'

since from work I have direct access to "B".
But from home I must do something like

rysnc -p N -a -e ssh --delete localhost:/tmp/ /tmp/

The problem is that there doesn't appear to be any such "port flag"
for rsync.  I just missing the flag among the gajillion or so
rsync flags?  If not, how does one do this?

Dean