Ah the fun of concatenated surnames, either with or without a hyphen.
Those things can cause a lot of trouble and confusion, even the registration authorities play it safe with such surnames, they often treat the first surname as a forename initial, and sometimes they also make two entries, one in that form, and one in which both surnames are listed.
They've done the former with your grandfather, his marriage registration lists him as Clark, John B.
Americans create the same sort of potential bafflement with their fashion for including a wife's maiden surname in her married surname.
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Anyway, your first task should be to give the UK GRO about £10 and get a copy of their marriage certificate.
http://www.gro.gov.uk/gro/content/You might get super lucky and find that he has listed his service number, although I doubt it, but there's a good chance that he would have listed his regiment or corps.
I'll predict that your mum got it right,...they usually do.
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This is their marriage registration.
The marriage registration entries for the bride and the groom are on two different pages of the marriage index ledger.
UK General Register Office for England and Wales...GRO Index Army Marriages...Army Returns - Marriage 1941 - 1945...
This is the bride's entry...
Terry, Dorothe V. married Clark...year = 1944...Country = India...Station = New Delhi...Page 245
This is the groom's entry...
Clark, John B. married Terry...year = 1944...Country = India...Station = New Delhi... Page 245
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Unusual spelling of the bride's first forename.
I'll have a poke around and see if I can find anything else when I have time, I'll give you a shout if I find anything.