Tillotson made a "J" series carb that was original equipment on Willys. They modified the "J" series into the "JR" series (the "R" meant replacement). Going from memory, there was a JR-1 through maybe JR-7. The JR series were basically "throw-away" carbs that were sold through the cut-rate mail order houses to customers too frugal to purchase a rebuilding kit for their original carb. All of the JR series were replacements.
I won't suggest that you not use it. However, some of the JR series have a cast iron throttle body, and we took several new old stock ones, and cut the cast iron throttle flanges from the rest of the unit. We could then weld two of the flanges together to make carburetor adapters. The rest of the units were trash-canned after cutting off the flanges.
To answer your question about the T (which has already been answered), yes it is an adjustable main metering jet.
EDIT: I just looked at the picture. The carb pictured is NOT a JR-5. All of the JR series were updraft carbs. Cannot tell for sure, but the pictured carb is either a DR or YR (again, the "R" meant replacement). The DR series was produced in the 1930's. I would have to pull the files, but I don't believe the YR series came out until after WWII. The DR and YR were downdrafts, the JR were updrafts.
To see a JR-5, see Ebay 291402291455
To see a YR-2, see Ebay 171722409076
Jon.
