It seems to me that in "open source model", "open source" is a compound modifier of "model" (some would hyphenate "open-source" to be clear).  Searching for "source model" seems to always find it preceded by an adjective ("open"), participle ("closed"), or noun "community" compounding with "source".  One instance where "source model" is clearly atomic is here, where "source model" is used to mean "gcc's internal model of the source code".  Maybe what we're really talking about is "license for use of object code" and "license for use of source code". Regards, —