What is trace flag 1605 in SQL Server 2008 R2? I haven't been able to find any documentation on it so far.
From an Official Source™:
It is a trace flag that was only used for testing purposes and is not enabled in retail builds.
So, you should feel safe to disable this trace flag and not impact behavior or performance. (I'd be much more focused on upgrading, anyway, if you plan to do anything with this system.)