The CW prefix makes it sound like a company or estate copy of a working or royalty plan rather than an NCB plan.
The main prefixes used in NCB south Wales area were:
R = both for early plans transferred from the old Mining Records office, and much later in C20 for over-size plans that were stored rolled in racks.
SW = used from mid C20 for south Wales area.
SWR = used from mid C20 for south Wales area over-size plans that were stored rolled in racks.
Plans were numbered sequentially in three main series, each starting from '1':
(1) early plans transferred from the old Mining Records office, mostly with R prefixes, highest numbers were only a few hundred if I recall correctly.
(2) the main series, from the 1870s to nationalisation when somewhere around 20,000 had been reached. This series included R prefixes where necessary for over-size plans. These R series numbers duplicated some ex-Mining Records Office R series numbers but they can readily be distinguished by their dates.
(3) after nationalisation, plans were numbered by area, so the SW series was started, again with R after the SW where necessary for over-size plans.
If your plan is SW630 then it would date from around the early to mid 1960s.
Note that plans were numbered in date order of receipt. So some old plans bear 'late' or even 'modern' numbers because they were only received years or decades after the relevant mines closed.
If you pm me a full description of the plan, especially mine name (if present - I presume not from your query), seam name (if present), company name (again, presumably not present) and date, then I'll be able to check it against the NCB catalogue of abandonment plans. If there is no date specifically stated in a title block, then there may be a date against a surveyor's signature, or failing that there are frequently very small dates within the workings themselves - usually in the format "16.10.47" where the last two digit number is the year. The date will be especially useful. If most or all of these types of information are not present then photos with a digital camera of the whole plan and close-ups of typical parts will assit in dating it.