Yes "T" is taper seat, which as far as I know also coincides with the small 5/8 plug. I've never seen a taper seat large 13/16 plug. Those have all been gasketed from my experience.

The "S" at the end stands for "extended tip" which has the electrode and ground strap extending slightly more into the combustion chamber which promotes better flame prognation. Performance oriented tuners generally switch to the extended tip plugs for better burn.

The S (extended tips) are all that I use in my stuff and actually if you cross over any AC R44 (standard tip) plug to an Autolite or NGK, you automatically get the extended tips with these plugs even though you're crossing a non extended tip AC plug.

Autolite 84-85-86 is the large 13/16 gasket plug with extended tips and the NGK XR4-5-6 are also 13/16 gasket plugs with extended tips, as well as the UR4-5-6 that Mr70 is using.

If you compare the stock AC R44 or R45 plug to any Autolite 85 or XR-UR NGK, you'll see right away the difference in electrode and ground strap distances.

Hope that helps.

I ran AC R43T's for 9 yrs in my 69 Z and that thing ran like a top!
It was 99.9% original/untouched except for a set of headers and it ran really strong and very smooth. Always started up with ease as well.