Pre-18th century British printing differed from modern practice in its treatment of the characters i, j, u, v, and their capitalized forms. In capitals, there was typically only one letter for what are now I and J (with several printed forms, some resembling a modern I and some resembling a modern J), and one letter for what are now V and U (with several printed forms, some resembling a modern V and some closer to a modern U). In lower-case, j is very infrequent and u and v are used based on their position within a word rather than for their phonetic value. In addition, w is frequently printed as vv.
The WWP encodes these letters as they appear on the page, with an <orig> element in cases where they differ from modern usage, and with the modern form recorded on the reg= attribute of <orig>. However, we do not hand-encode them; we have an semi-automatic routine which assists a human operator to locate spellings which should be tagged with <orig>, and auto-tags them if appropriate.
We do not use <orig> within <mw>.
For more information on different forms of these letters, and what they represent, see 109.