NL,
The 'original' GSM was issued twice IIRC first pre WWII and secondly post WWII up to Dec 1962 when Brunei was the last clasp. Not sure how many clasps were issued for both.
Your MC versus DFC might relate to when the DFC was instituted and remember, the RFC was an Army thing!!
TTN will have the full details - I shall defer to him.
Old Duffer