AFAIK this is true since the Sigma Pro remained a custom order headphone even after it had been "discontinued". Stax never keeps a stock of old drivers if they produce something newer so a later Sigma Pro would have gotten LNS drivers or even 404 driver.
The Sigma/Lambda drivers are identical, only the protection grill is mounted to the opposite side of the drivers in the Sigma's. The core unit is symmetrical so it has the gold anodized rings on both sides.
Maybe looking at the drivers from the inside, the Nova driver may have golden ring around it while the Signature has a black ring. But since the drivers are mounted differently in the Sigma shell, they might have made the drivers differently and it that way it might be impossible to tell.