So the rule of thumb is the barrel is 3 months or so older than the receiver (barrel 6/40, receiver 9/40). Or would they be closer?

I came across a 1940 barrel and want to look for the close receiver via date. I know trying to find the other 1940's parts will the costly. So I figured I have the barrel, get the receiver and than the op rod and bolt. Don't worry about the rest as it may take some time and money.