After I had written my previous screed it occurred to me that somewhere, (did you mention it?) he was recorded as Marks - not Mark, which to my mind would not be very Jewish so I wonder if he played around with his given name as well as family name - could it perhaps have been Marx or Max (Maximilian)? Another point is the eldest son. It is not only that he was born in Russia/Poland but there seems to be an enormous gap between him and the next sibling - a second marriage? Finally I would suggest that you try to google up a map of the boundaries in the mid 19th century then at least you can picture which bit of Poland was in "Russia". I did try to look for his naturalisation application because in one of the Census returns he does claim to be a British Subject - but no luck, although that could also have something to do with name changes. Anyway - best of luck you look as though you have a task for life! If I have any more ideas I'll come back.