Luckily I happen to have a disassembled 603 engine in my garage. After looking at your excellent pictures and reading the replies to your post, I agree that the most likely source for your oil leak is from a bad valve seal or guide or combination of the two. You are right that it isn't coming from an external source--you would either see it, or it would be spraying under pressure.
In thinking of a simple way to see fur sure, I came up with an idea that might work.
Pull the valve cover.
Sop up amy oil in the valve spring area around the exhaust valve in question. Blow it out if you can't reach it due to the spring.
Spray a liberal amount of a very light weight oil directly on yhe valve stem and seal (penetrating oil or such). make sure it isnt some liquid that will damage anything.
Look under the exhaust manifold and see if it comes out.
Since none of the rest of the engine will be receiving oil, this will be your culprit if the penetrating oil leaks out.

I'll put my theory to test tonight in the garage. I'll put a valve back up the guide to see if the oil runs down and pools right where the exhaust manifold meets the bottom of the port.
Steve