[Griffon]

Hello David!

I do not know if this is of any help for you, but I do own a so called "Luft Naviations-Karte in Merkatorprojektion" and checked the grids you provided on this map;

grids 12 and 22 (I would start with these as a reference for the coordinates you have given) lie on a straight line between Vlissingen and Southend, grid 12 contains Canterbury and Folkestone in its South-Western corner and runs up to Harwich while grid 22 lies to the North West of the Straits of Calais and has Dunkerque and Ostende on its Southern edge.

I do hope that this is a starting point for you, unfortunately I cannot tell you how the grids on this navigation chart are subdivided so I cannot provide more details;

cheers from Vienna
phil