They are likely the same as used in modern OS maps - <a href="https:&#x2F;&#x2F;www.ordnancesurvey.co.uk&#x2F;resources&#x2F;maps-and-geographic-resources&#x2F;map-abbreviations.html" rel="nofollow">https:&#x2F;&#x2F;www.ordnancesurvey.co.uk&#x2F;resources&#x2F;maps-and-geograph...</a>