Tremec Mainshaft Small Tapered Bearing & Input Shaft Bearing Race. #11 & #22 on the T56 Transmission Disassembled View, seen here, here and here.
This is the 'pocket' bearing that presses onto the front end of the main shaft and rides against the bearing race that presses into the rear of the input shaft. It is found in this location on ALL T-56 transmissions and also TR-6060 transmissions in this same location until 2012 model year for OEM TR-6060's. It is also found in standard T56 Magnum transmissions. The Magnum XL, new Magnum F, and 2013+ TR-6060 transmissions use a later design.
In addition, this bearing is also used in T-56 transmissions that have a two piece cluster gear design on the rear of the cluster extension shaft and the race slips into the tail housing.