Surprising as it may seem, the Excalibur conversions of Piper Twin Commanche and Beech Queenair aircraft were developed and sold by
Ed Swearingen of Swearingen Aircraft, manufacturer of the Metro. They were based at San Antonio, Texas, with an additional assembly facility in Martinsburg, West Virginia.
The only Queenair Excalibur I knew of in this part of the world was the one operated by Sunflower Airlines in Fiji, but there may have been others.
de Havilland in Sydney did Reisbeck and other King Air conversions, so I guess they
may have been able to carry our Queenair Excalibur conversions.