When was the last ports tree that worked with RELENG_6?

[Darren Pilgrim]

I have an old 6.4-R machine which I can take as far as RELENG_6, but no 
further due to a hardware-support issue (special hardware, third party 
driver).  The hardware needs to live a while yet, so I'd like to try to 
get the installed ports as up to date as possible.  The current ports 
tree doesn't work on 6.x, so I need to find the last version of the 
ports tree that does work.

The ports tree on the box is *newer* than the one tagged RELEASE_6_EOL, 
so it's not that simple.  The current tree was pulled 2012-Feb-21 10:38 
UTC and I know it works.  It looks like the RELEASE_8_3_0 tree 
(2012-March-7) also works based on some very basic testing.

Does anyone when the ports tree broke for 6.x?  Alternately, if someone 
knows what it was that (first) broke the tree for 6.x, I can easily hunt 
down commits and find a predated tree that works.  Was it the optionsNG 
stuff?