Thanks for clarifying that John. Even not that many years ago I know of some warranty W motor 409 short blocks that were found in storage at old dealerships.
At a large auction - attended by people from all over the world - several years ago near where I grew up, there were all kinds of service parts that were sold, including such things as NOS crankshafts, camshafts, heads, etc.
Dealers talked to each other for such things as trading cars - and parts - in the 60s and 70s. So it wouldn’t be a stretch to think that a few short blocks were obtained that way.