There's a shaft in a shaft there so there's a outer typical shaft seal and another O ring type seal inside. It aint easy to tell which is leaking but we know which has seen the most movement. I would replace both.
When you drop the pan you'll discover a whole bunch of bolts in that valve body. Most of them are shorter and holding the valve body together. A few are longer bolts are holding the valve body and filter in place. A service manual would show the few you need to remove.
I did all this to my 67 904,,, and the front seal start leaking about 3 months later. Stands to reason since they are all the same age. Better luck with yours.
