I strongly suspect that at birth he was registered as Mark Winter, born Wellington 12 Aug 1851, twin brother of Josiah (aka Joseph). In 1861 the boys were living with their widowed mother, Ruth, in Wellington.
I am in the process of making as many on-line checks as possible to test this theory before purchasing a copy of the James Mark Winter/Fanny Gamlin marriage Bristol 1873. This should confirm (or otherwise) my deductions.
If I am proved correct, my next challenge will be to determine what happened to him and his family after 1881. In 1891 I can only find his youngest child, Ada Lillian, living with (presumably) her maternal grandfather, Thomas Gamlin, in Milverton, Somerset. Clearly emigration is a strong possibility, but why leave Ada behind?