To Pierre    I once had a friend in the 70's that owned a GM car. The name on the front was "ASTRA" (Pontiac) but at the back it read "VEGA" (chevy)! I think you have the luthiers equivalent. The factory either attached a Tokai neck to a Sigma body, or picked up the wrong branding iron by mistake. It certainly gives strong evidence that Tokai was a manufacturer of the Sigma line. I have always thought my early DR-7 could have been made there.
A very unique instument of obvious high quality.

Cheers   Neil