Seems confusing but there are plenty of lawsuits out there that explain why its considered counterfeit when you sell a Rolex that has a custom part or is altered or modified with non-genuine parts. For example, in the lawsuit against Melrose Jewelers, Rolex sued them for Trademark infringement and won an $8.5 million judgment and shut them down.
Read it here.
https://law.justia.com/cases/federal...00/538474/105/
extract:
"Melrose offers for sale and sells, among other items, altered Rolex watches that bear one or more of the Rolex Registered Trademarks. Melrose s altered Rolex watches contain one or more non-genuine components (i.e. components not manufactured by Rolex, including, for example, non-genuine bezels, bracelets, and/or dials.. Melrose s altered Rolex watches have non-genuine bracelets that bear counterfeit copies of Rolex s CROWN DESIGN ( and/or ROLEX marks."
A Rolex with a non-genuine or altered part is considered ( the watch ) counterfeit - it is a fake. Even if its been disclosed.