Multiwire branch circuits back in the day

[mopowr steve]

In two of the three code sections that permit a common neutral, no guidance is given as to the sizing of that conductor but 227.7(B) says this:

The rule above may actually permit the #10 neutral, if we knew the loads as is says the neutral is sized by the load and not by the OCPDs. With unknown loads it would make some sense to size the neutral for the circuit in question at 40 amps.

Well, I'm looking at a 2011 handbook. Commentary below 225.7(b) points us to 200.4 which states "Neutral conductors shall not be used for more than one branch circuit, for more than one multi-wire branch circuit, or for more than one set of ungrounded feeder conductors unless specifically permitted elsewhere in this code."

So the only permissions are for 215.4 Feeders with Common Neutral Conductor and 225.7 Lighting Equipment Installed Outdoors.

Thanks everyone for helping me to determine what was/is/should be done!
Not easy when dealing with rare instances.