Stage I vs Stock
Per the Buick Power Source-
Better material, iron is alloyed to 42,000 yield, vs 35,000 stock
(the Power Source has confusing words here)
Thicker deck face
Material added to allow for extra head bolts- drilled on Stage II, not on Stage I.
Thicker center main bulkhead, drilled for four bolts only on Stage II
There's more, but basically, the Stage I block was used for the Stage II, with the added machining done.
The E-bay sale item appears to match the description of a "fitted block", rather then an engine assembly, either short or long, or maybe an "engine, less cmshft"??