I've not been able to make heads, nor tails of the numbering system. I do have a suspicion the numbers may have to do with sizes of wheels ground on, but I dunno. Also is the Zwillingswerk and Twinworks' numbering systems different? I have a #66 which is 6/8 and half-hollow and a 15/16 #66 which is the thinnest Henk I have ever owned. I would like to know myself. One thing is certain. In good shape, they ALL shave great!