The input shaft of the transmission.
Just the output shaft of the transmission.
23 Splines. See the Novak NV3550 knowledge base. http://www.novak-adapt.com/knowledge/nv3550.htm
It measures the speed of the output shaft.
on the output shaft of the transmission, close to where the drive shaft enters
Manual transmission that is in gear-YES
A gear which causes the transmission output shaft to rotate at a higher speed than the transmission input shaft.
Most 3 speed manual transmissions without overdrive have 2 shafts, a mainshaft and a counter shaft or ( jackshaft). The main shaft is the shaft that the drive shaft is coupled to by a universal joint at the output of the transmission. When the transmission is in 3rd gear the output of the transmission is the same RPMs as the engine or the input of the transmission. So in 3rd gear or 4th gear in a 4 speed the mainshaft becomes as one from the input from the clutch to the output to the driveshaft
The output shaft seal has failed and transmission fluid is leaking by, and that would explain the transmission fluid appearing there.
It means that it has at least one gear where the output shaft rotates at a higher rate than the input shaft. So, if you have a transmission with a .70 overdrive ratio, it means the transmission output shaft makes one full rotation for ever 7/10ths of a rotation from the input shaft.
When the input shaft is locked to the intermediate/output shaft
most likely worn output shaft bushing worn