Here's what Jonathan Eastland has to say about this:
"In a letter from Leica GmbH to the author, dated 27 April 1993, a company executive stated that neither Leica or Leitz ever smelt its own glass from raw materials for production purposes."
He goes on to explain that Leitz used to operate its own Glass Research Laboratory, but it was not for production purposes. The lab was for exploration of new glass types. Production glass is smelted to Leica's requirements by outside sources.
Reference: Jonathan Eastland, Leica M Compendium. Hove Books, 1994.