From memory, the Packard Merlins were just the same as their U.K-built counterparts, except that they used U.S. sized nuts, bolts, threads etc. The engine designations were similar too. The Packard Merlin version of the RR Merlin 66 being a Packard Merlin 266, and so on. Otherwise, there were very few differences.
A MkIX Spit' produced at Castle Bromwich, but fitted with a Packard Merlin 266 simply became a Spit'XVI. Externally, the a/c looked the same.
