Mount Zion Chapel Marriages

The Calderdale Archive Service indicates the following regarding researching earlier marriages at Mount Zion:

Even though copies (the main ones required by law) of Mount Zion marriage registers would be with the General Register Office, the duplicates which (presumably) would have been kept at Mount Zion at one time, have obviously not been deposited here at WYAS Calderdale (this is not a legal requirement).
This is a pattern often repeated with nonconformist places of worship: even though marriages were licensed to be performed from one date or another, marriage registers do not always survive to be deposited by enlightened individuals in archives such as ours.

One assumes: they were simply lost
OR they were kept in private hands
OR duplicates were not kept (we are not sure of the requirement in this respect)
OR duplicates were handed in with the main registers to the GRO at a subsequent stage (sometimes, after a ‘sort-through’ of things at the GRO, duplicate copies which are found there are sent to us, but this does not always happen).

Many gaps thus exist in the post -1837 nonconformist marriage records held here, but these gaps would not exist in the GRO record.

If you would like more details of Mount Zion marriages, the General Register Office would be a good place to start?