My W115 71 220D where the engine is actually a 1973 220D has a camshaft # that was not listed in the manual too. It might have been a #18 as well. I was trying to make sure it has the correct camshaft for the valve rocker arms and nuts (soft vs hardened), but never was able to figure it out.

On the chain situation, when I had the engine apart, I closely inspected my chain and saw 2 tiny metal shavings embedded in the chain links and preventing the chain from swiveling the way it should in two spots, so I had to replace it. If it hadn't been damaged, I would have reused it with an offset key (about 6 deg. stretch).
Against everyone's recommendation, I decided to use the beck/arnley chain. The reasons (not necessary in order of importance):
1. It is a continuous chain and doesn't have to be crimped (obviously it can only be installed with the crankshaft and all guides out of the block).
2. It is made in Japan.
3. It costs $45 shipped.

The IWIS chain may be better, but it costs $120, the crimp tool is very expensive and there is the chance that you might not crimp it correctly.

That is why I would vote for using an offset key unless you are doing a full rebuild, but that's just me

Now a question for you - You said "I've adjusted the IP and linkage to compensate for the lower air density here."
can you elaborate on that?
I am in Salt Lake City (4400 feet altitude) and I get gray smoke when first taking off in 1st and halfway trough second, and through 3rd and 4th I see some black smoke when accelerating. My diaphragm spring had 2 thick and 1 thin shim, I removed the 2 thick shims and the smoke seems much less - I assume that the same amount of vacuum will now be able to pull the rod a little more toward less fuel. What else is there to be adjusted? And what about the ip linkage? Maybe the tickler shaft engagement - it is half ball length right now (per FSM), maybe increase it to full ball's length so that at idle and low throttle, the tickler is slightly more pulled back and applies less fuel?