Small loose fitting rubber vent hole cover.?.?? But it should not be on OD. Maybe its on the OD adapter between the OD and the trans? A possibility of a picture?? OD gets fluid from trans through holes at back of trans case.

Solenoid: Suspected electrical problems. And you want to be sure about electrical continuity from driver action to solenoid. REPLACE the 4th gear switch now. Since it is so hard to get to, is it worth the $$ now to not pull the tranny to replace it later. You did say you were about to cut the tunnel to fit...

Test solenoid. J type. on OD. Apply power through Amp meter. Draw should be 2.2 amps or so. Solenoid should click with sharp metal hitting metal sound both on and off. Other alternative. If solenoid on unit in auto will be impossible to get a wrench on it, even the "special" wrench to fit the tight 240 tunnels, remove solenoid and "rebuild" it.

E-mail me for a set of instructions for solenoid rebuild.

Gear lube. Manual trans before the M45/46. M45/46 and up use Type F ATF. I've been taught gear lube is near 30W oil with lots of sulfur.

Duane