Here's the link to the
Stewards decision in full
I hadn't appreciated the decision doesn't concern itself with when the transfer of information occurred. The rule on listed parts is firm, both brake ducts on the 2020 car must be designed by the constructor.
What the Stewards have subjectively said, is it would be unfair to disallow parts that have been previously incorporated in cars of the past and have significant effort invested in them by the constructor (this is the grandfathering-in process). It's somewhat of an unusual approach, as they have used a "spirit of the regulation" interpretation in determining when a design become as much one team as it does another. The stated aim of the regulation being to prevent a constructor gaining an advantage by short cutting the design process
With the front brake ducts, the test is met due to the investment of sufficient design effort during the 2019 season. Racing Point extensively copied the Mercedes design for the RP19, and then developed that design as the season progressed. Essentially, the front brake ducts at the end of the 2019 season were as much Racing Point as they were Mercedes and the incremental development of the 2020 front duct has not short cut the design process
The rear brake ducts, however, the design process was started in late 2019 and was specifically directed towards the 2020 car, and as such Racing Point should not have relied on the design information previously obtained from Mercedes, as that did short cut the design process
4. If Racing Point had asked the FIA at the time (September‐ November 2019) if it could use the CAD
models of the Mercedes W10 RBDs as the basis for the RP20's RBDs, the FIA would have said
definitely not, because (in contrast to the position in respect of the FBDs) Racing Point was not
refining a component that had already been incorporated into the DNA of the RP19. Instead, it
was introducing a completely new component for the RP20, which it knew was classified as a LP
under the SRs (because they had been issued on 30 April 2019)