What is this Mission-10dd and Mission-10ch numbering?

No, Mission-10ch isn't necessarily a variation of Mission-10c and neither is Mission-10dd a derivative of Mission-10d. It's just that 'ch' is the letter after 'c' and 'dd' is the letter after 'd'.

It's simply that we're going up through the Welsh alphabet as can be seen here.

If we ever run out of letters in Cymraeg then there's always the characters at the end of the Norwegian alphabet to get stuck into.