This is always misunderstood.
There were the old original identifying numbers. They were called the IM or InterMix numbers (I hope I got that correct). The color tint formula changed with each change in paint types and the type of tints. The manufacturers carried forward some equivalent tint formula for mixing the old colors with the new tints.
If you call the manufacturer or go to the dealer they should be able to find the formula for you. I called PPG and they gave me the original tint formula from the 1930's and they could tell me the current numbers for the modern paint they sell.
What you need to consider is how close to correct original do you want?
What they carried forward may not be exactly like the original. I can tell you that the PPG Concept Lombard blue is too light. The new formula includes a bit of white and I would say that the white should not be in the formula.
I also had talked to BASF or the paint line they sell. They gave me a very wrong mix of tints for the lombard blue, it would help if there was a blue in the formula.
It has been my experience that both Dupont and PPG will talk to you quite a bit about their paints if you call their 800 tech number for automotive paints.