Oddities such as this do pop up from time to time and I really have never heard or seen an 'Ah! That makes sense'
reasoning put forth ...
* the "Secret Test Engine" theory
* the 'Reworked' due a line worker mistake and then snuck back into the assembly line theory
* the 'Lost' in the warehouse theory
* the 'Warranty Return' and then repaired theory
* even the Chevy used it to race and then returned it to the assembly line theory
Non of these is plausible or even possible
Development or 'Test' engines - other than a unit selected from the line to do a sample data run or other minor QC check - were requisitioned by the engineering departments and were encoded (stamped) on the pad with an identifier code that included the Work-Order number traceable to that group or department.
These parts were closely tracked and often destroyed after the work or tests were completed - sometimes even during the test
There were/are lots of different coding systems depending on the Part(s) be used, Department and/or Intention of the parts (Prototype/pre-production/production testing) being modified or developed/tested - no need to go into all of them here, there's been numerous publications with the info in them ...
All Warranty 'Adjustment' engine parts were destroyed under GM requirement - that's not to say a block or other parts didn't sneak out of a dealers back door at times - there's something in my shop to prove that 'could'
There were also some parts released for "Field Testing" and/or 'Evaluation' (remember GM DID NOT
Support any Racing during these years
... ) that found their way around through the years and have ended up in some pretty interesting places
Sooooo - I would guess we're just gonna have to follow that great philosopher Forest Gump is saying "Stuff Happens" and let it go at that.
Remembering - of course - that these are THE EXCEPTION, never the Rule