Searching around RadioMuseum, I see there's a third model of blue-mirror sled that I've never seen before, in addition to the 557 and 558.http://www.radiomuseum.org/r/sparks_wit_457x_457_x.html
The one pictured apparently crossed German eBay.de in 2007. And there's a photo of an article about one found in France in 1987.
Does anybody have one, or has anyone ever seen one in the flesh? It was an export model. Must be quite rare. Apparently it's never been mentioned on ARF before; I didn't find any previous threads about it. The only round-dial Sparton sled!