The rear output flange seal is installed in the rear case before the flange is attached to the output shaft.
The seal case has a dust lip that prevents existing seal installers from fitting it.
GM-49861-A installs the seal without damaging the dust lip, and can be used in place of GM-49861 in current 6T70 applications.
Failure to use GM-49861-A will damage the seal or dust lip.