[b]Steve:
ORIGINAL: ysteve1
...how can I tell if I already have the "late type" carb?
A simple answer: "Damfino."
In appearance all the parts look the same, but I've cured a few engines with the upgrade. Might be the taper of the needle, or the size of a drilling, possibly the spray bar has a slightly larger outside diameter, Could be a combination. I've really never done a close comparison.
It's easy to tell if you have the first series TN carb though, it is the one that actually uses a LS needle. Second and third series have the sleeve metering.
The first attached composite has the mid block LS needle type on the left, and the big block sleeve meter on the right. Stuck this one on to show spray bar differences, the mid block has all the fuel going out the end of the bar, the LS adjuster (needle or sleeve) occludes the end to control the flow. The big block has a slit in the spray bar, the needle or sleeve moves to restrict the size of the opening.
Second composite shows all mid block parts, sleeve type on top. Don't be misled by the choke on the needle type, the choke can be used on either. Second and third series carbs look the same; both sleeve metering.
Bill.