Packard was in a better place to do it as Packard was buying (or making) all new machinery. RR needed to introduce the new parts/processes to existing production lines without interrupting the flow of engines.
Packard did change a few things, like some of the connections for the coolant passages?
It was pretty much a co-operative effort.
- RR engineered the 'two piece block'
- since Packard was to start making Merlins, the prudent thing was for them not to make any of the 'single piece block' engines, but to start with the 'two piece block' engine
- RR followed suit quickly enough (1942?)
I hope Snowygrouch will lead us through this