Not sure how to answer you other than "one form a silver faced concert lead", which sounds rather sarcastic although it is not meant to be.
The concert power transformers are not an off-the-shelf type of item. Sunn probably had them made to their specifications.
If you really need a power transformer, your options are somewhat limited.
o You could call around to various repair places that did warranty repairs on Sunn equipment in hopes of them having a spare NOS transformer. But this is probably a slim to none prospect.
o You could contact a transformer rewinding service to see if they could repair/rewind your existing transformer. You might find one, but chances are it would be expensive.
o You could find another non-working concert with a working power transformer. This would be my option.
