I see no obligation to distribute source to parties that are not in the distribution chain. If you allow public downloading of binaries then each download is a distribution and source must be available for each download. If you distribute to specific parties then your obligation is to those specific parties but not to the public.
Section 10. Automatic Licensing of Downstream Recipients.
This section releases the distributor from obligations to enforce the GPL terms on third parties.
This may be nit picking but it is important to know what you obligations and rights are.
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