If they don’t have access to analog or digital versions of the original stems, I believe they will master the already mastered audio. This would be rare though. They would usually have these things available.
I imagine most big studios/labels have a process for digitising analog recordings for archiving and future use, and I would think they’d have a dedicated team working through it. This means they would probably have these things on hand or at least quite easily accessible when needed.
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