As far as I know the Merlin always had a two piece engine block. The difference, some people believe , between the Royce and Packard versions were that the Packards were built to American automobile tolerences which were far closer so that component changes were just a 'pull out, slot in' job. British engines always required individual selection and fitting.
This was also apparant in the British car industry in the 40s and 50s. Cylinder block would have 0.20 or 0.10 stamped by the bores to indicate what oversize piston to fit; same with the crankshaft. You never had to worry about that on American engines.