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[Comberton]

It must be a mistake as baptisms for both sons show parents as Edward Clarke and Marian Gertrude.

[bramhallbill]

The son died in WW2 Perhaps he had never met his step mother or his wife forgot. Mistakes do happen

[Tickettyboo]

The GRO Birth Reference always gives the maiden name of the mother eg the surname she was born with and which she used prior to any marriages.

With the caveat that the GRO gives the maiden name of the mother - as far as the informant is aware.
I have had a few cases where the mother was either previously married, or had been brought up being known by e.g. a step father's name. Depending on who registered the birth and what they knew about the mother's history, children were recorded with different MMNs.

Boo