Each full 6" sheet is divided into four quarter sheets: NW, NE, SW and SE.
Each 6" quarter sheet is divided into four 25" sheets. These are numbered in four rows of four sheets within each FULL 6" sheet: top row runs [6" sheet number].1 to [ditto].4, then next row [ditto].5 to [ditto].8, then next row [ditto].9 to [ditto].12, and bottom row [ditto].13 to [ditto].16.
Thus quarter sheet [ditto]NW comprises 25" sheets 1, 2, 5 and 6, and so on for the other three quarter sheets.
For example, 25" Cornwall sheet 10.11 is the NW quarter of 6" Cornwall sheet 10SE.
1:1,250 sheets (rather uncommon) were quarters of 25" sheets, numbered NW, NE, SW and SE: e.g. the SE quarter of the example given above is Cornwall 10.11SE.
1:500 (ten feet) sheets were 25 sheets to a 25" sheet, numbered in five rows of five, from 1 in the extreme NW corner to 25 in the extreme SE corner. E.g. the central 1:500 sheet within the example 25" sheet mentioned above would be Cornwall 10.11.13 (note that this is probably a rural area and so was not surveyed or published at 1:500, I just used this as an example).
David Archer Maps sells inexpensive reprints of the index sheets for all of England and Wales in A4 paperback book form; he has a website.
The authorative guide to historic OS maps of all scales is R.Oliver "Ordnance Survey maps: a concise guide for historians", published by the Charles Close Society for the study of OS maps - 256pp hardback, about £25; the society has a website. The book includes details of the dates of every survey and revision of each county series set of 6" / 25" / 1:500 maps with details of which sheets were and were not revised / published. When read in conjunction with David Archer's book, it'll give you authorative information to answer your query.