I have been collecting engine numbers as part of my Aurelia Register activities and have something to add to the discussion, although not conclusive.
NON-STANDARD ENGINE NUMBERS
The following are the few non-standard numbers I have recorded. Some are likely mistakes made in locating the engine number & maybe casting numbers found on the engine.
Francesco Gandolfi offered some notes in an email, 22-23 Sept 2014:
"Not all engines went onto a car, maybe some were sent to dealers who had sold a car which quite quickly showed engine problems, & the original was replaced without this showing on the production records". FG also notes [some editing by PM]: "..Franco De Virgilio once told me that the replacement engines were not numbered at the factory& that this was not recorded in the Production documents as would have been a long & expensive procedure. When a dealer needed an engine to replace one which was too expensive to repair, he would inform the factory of the number of the engine to be replaced, & the new unit would have been engraved with the same number. This however, was only in Italy, but I am positive that importers/distributors abroad kept a stock of spares." FG then agrees with me that numbers such as B20.0011 may have been stamped by the local distributor when a replacement engine was supplied under warranty.