This is a very substantial mains isolation transformer suitable for use powering a whole hi-fi system if required. Whilst there's no details on the unit itself, I believe it is a factory made Ben Duncan/Russ Andrews product and given the size and weight of the potted transformer I'd guess at it being rated at circa 2000VA (or possibly quite a bit more - it's heavier than an unpotted 4kVA one we have here). As can be seen from the pictures it has Russ Andrews hardware on it (Super Clamp mains plug on a short cable and double Super Socket output) and is of an unbalanced design.
I'm a big fan of using isolation transformers, as they prevent DC passing through and can have a filtering effect for mains borne high frequency noise, amongst other benefits.
There are some marks/chips on the casework, but given it's a box that's likely to be hidden away I doubt that will be an issue to most owners.
