Well, the EPC shows part number 601-011-03-10 as a cylinder liner / sleeve for the 603.970, .971, and .972 engines. That sleeve is not used on any of the 602.96x or 603.96x variants. But yes, the sleeve was omitted from the factory... however the FSM does indicate that is normal.

Now, how practical it is to sleeve the .97x is a different question. But with siamesed cylinders, would it really be a problem with only 4mm separation? After boring & sleeving, it would still be 4mm, right?