Could be, there were a lot of catalogue numbers set aside for which there seems to be no information today.
After just digging out "The Motown Collection", the previously-unreleased tracks seem to be mostly recorded between 1965 and 1968, but spread out pretty evenly across those years. I don't see that, for example, there were a bunch of these tracks recorded within a single month or two, as one might expect for a planned LP.
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