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For the time being, most or all of the chips that I am working on are from old Motronic 1.x engine control systems dating from the late 1980's through the mid-1990's.
For now, it would seem that Bosch's internal part numbering is somewhat indicative of the chip manufacturer. 3xxxx series devices seem to be custom Bosch parts, and 5xxxx parts are parts made by other manufacturers (either re-branded standard parts, or possibly customized versions of standard parts). This numbering system may only apply to old parts from the 90's since that is what is in the collection.