The man I asked (who is an experienced engine-builder) confirmed what I thought after seeing the photograph: that main-bearing cap looks like steel rather than cast-iron.
In which case the code will indicate some aspect of how the block and caps were line-bored as a set prior to assembly (and the significance of the characters will depend on the machinist / machine-shop).
Do you know the provenance of this engine? Was it built for competition?
This is getting beyond my experience and expertise, so perhaps it would be worthwhile contacting a good engine-builder. Someone with whom I occasionally correspond has been happy with Seven Mini Parts in Auburn, CA, but of course you may have more convenient / preferred contacts.
Good luck and do let us know how it turns out.