Date: August 07, 2008 at 01:02:58 From: jhadhar65 Subject: Re: I think it is for RFI shielding.(NT)
Sounds like you're talking about the flux band. It's there to help reduce/contain stray magnetic field leakage (EMI). They're more common with power transformers rather than output transformers, although I don't know why an OT couldn't make just as much use of one. Core straps on smaller transformers that are used for two-bolt mountings do the same thing.
It differs from electrostatic shielding in that the flux band doesn't need a separate ground - it gets it from the tranny body being bolted to chassis.