Thanks again guys, this is really intriguing. So, the guy that I am studying was rated as 'Category C' by the OC of the MOTU on 17/12/1964, The next two qualifications cited in his logbook are simply stated as 'B' on 29/01/1965 and 'C' on 01/02/1965 - both are signed off by the OC of No.201 Squadron. A further simple 'B' entry appears on 07/02/1966, again signed off by the OC of No.201 Squadron. He is then signed off as being 'Category A', during a visit by the CCCB on 18/02/1966 - all of this being while flying Shackletons. By 18/08/1972 he was flying Nimrods at No.236 OCU, being signed off as '18 GP D Category'. This, to me, suggests that the simple 'B' and 'C' entries are not ability category ratings but, if they weren't, then what were they? They must have been some type of qualification, as they are listed on the 'Special Flying Qualifications and Renewals' page of his logbook? Can one of you knowledgeable people on this forum please help??? Thanks again, Andrew