The pics I'm viewing that I think MarkT made viewable of yours it shows the outer case still on. Maybe I'm half asleep 'normal for me lol' but if I'm viewing correctly that outer case needs to come off to get to where you need to be. 

Thank you RT325!  You're correct.  I have an old tiny phone and did not notice that!  Looking on a computer it is clear you're correct!  Great catch!!!!

(Also that bike is likely a 1973 LT3 which can be verified by the VIN.  Some parts like the air filter are different than the LT2)

Oh my, are you saying the shifter oil seal is behind the left engine cover??  I thought it was supposed to be visible to the outside like the kick shaft seal is.  

Yes.  The front sprocket/ignition cover must be removed.  

I can't believe I didn't catch that!  Glad we have some real experts around here!
 

Holy smokes, after I already got the right side cover removed!   Ah well, I needed to check on the clutch anyway.  

Not at the bike now and currently with a frosty beverage in hand - does the rotor/stator need to come off for that left side cover to be removed?  

No need to touch ignition, and removing the right side was hopefully not a waste of time.  I've had to remove the right cover about 80% of the time because 2 out of 3 times I manage to disengage the shift claw somehow.

The thing I'm wondering more about now is if it is even is a shift seal.  Could be sprocket seal or even a broken plastic shift drum retainer cover.  

All good MarkT. A learning curve N811. If it's been parked that long the plates might need seperating as stick together but if you've had it running & it's working then you're OK. But as Nark says the claw arm can become detached. You could look over or past the clutch to see it behind but if changing is working then it's in place. Might pay to check the seal on the change shaft before re-fitting the right side cover incase the arm jumps off while fitting the seal. Might be a sprocket seal leaking or sprocket nut undone. Or worse there's a three screw plastic cover above the sprocket that gets damaged if chain derails. Another thing 'is' people fill the Trans to the top so it forces it's way out anywhere it can & out the trans breather pipe also. Full isn't super full though as the filler is low-ish by memory. Sorry, short text went long didn't it lol.

OK, so the game plan has definitely changed! You both mentioned the plastic cover (apparently called a blind plug) and sure enough it's been smashed up - not to mention a whole nother world of crud under that left cover. I'm going to assume that is the source of the gusher for now but will go ahead and replace the shift shaft seal anyway (which I can now clearly see!!). Thank you all for your help!

Pictures (if I can post links yet) -

Still can't post links. "https:/" plus "/1drv" plus ".ms/f/c/19efa8465f6112d0/EkFAKSETaUZAg0LuRmpzByABAx5cYBMnuNfmsGPWvjZUXw?e=srdK56"

Also - definitely not a waste of time removing the clutch cover. Lots of deep grooves in the basket, so going to replace that and all the friction plates while I've got it apart.