[Beardyman]

Mr Moose,
Now that makes a lot of sense. It would go a long way to explain the smaller speaker & "Baby Grand" front panel! Well done sir!
I'm guessing the original 52 chassis went bang at some point & was replaced with a 55/56? We will never know for sure but as other have said, these were expensive bits of kit in their day so it would have been worth doing.
The picture from the link looks identical to mine although the schematic does have some differences from the 55/56 model. I shall delve deeper into the mystery at the weekend. Many, many thanks!