Are these 3 screw carbs for the early cars?

http://cgi.ebay.com/ebaymotors/Datsun-240Z-early-SU-carburetors-/220740326878?pt=Vintage_Car_Truck_Parts_Accessories&hash=item33652615de

Over what period of time were 3 screw carbs used? I thought someone might be interested in these...

Thanks, Charles

3 screws were the last carbs used before the infamous flat tops came out.

3 screws were used in conjunction with the manifolds that had the water ports between the bottom two mounting bolts on the inlet side of the manifold.

Functionally no difference between them and their 4 screw brothers.

Thanks, Bruce

I was looking through the spotters guide after I had submitted this thread and that is what I understood. I had always been under the impression the 3 screw carbs were an early car thing. Also thanks for the email last week I will be pulling a set of carbs together for shipment soon.

Thanks, Charles