The Turbostar 540 is in effect a Turbostar 432 running at 5.33 MHz, rather than 4 MHz, with the Kasparov Selected Openings module built in. This machine sold in the USA in small numbers. I have not seen a photograph of it but believe that it looks (very) similar to the Turbostar 432. It was supplied in a Turbostar 432 box. A white label was added to the back to distinguish the 540+ from a 432.
(My thanks to Albert Rine for this information)

In the 1986 Fall/Winter edition of the American publication Chess Computer Reports (Pages 79-81) it was reported that SciSys would be developing further models of the Turbostar range, to be known as the 640 and 740. So far I have seen no evidence that these models were produced or released for sale.