These are not my area, but the 6B/ prefix denotes it was in the Air Ministry's stocks. I am not sure what use the Air Ministry would have had for a sonar timer during the war. Does anyone know when aircraft dropped sonar buoys were developed and first used by the RAF? Perhaps it is related to that?

One person who I would recommend that you try to contact would be Craig Pearce who sometimes drops by this forum. He has been making quite a scholarly study of British Empire and other military clocks and watches. Here is a link to the 'non clock, British, aircraft timepieces' page of his website One of Craig Pearce's tables - the one with this RAF code listed on it where he makes available his table of British second world war timepiece markings that he is working on.

Great infos, Jim, I thank you. Well its seems that there are many things to explore ... but that's the blood of our passion, isn't ?

About sonar buoys I remember something, very vague, that surprised me because I thought "oh, so early ?", it was 1943-44, in any case before 1945, in "Seek & Strike", a book about ASW in WW1 and 2. I'll check, it's up in the height shelf of the library, but I know where. If I'll find anything uselful I'll return on you.
Thanks again for now

Yes I do have a look at the forum from time to time.
I have been chipping away at finding the answers to a few questions I have. I come across all this other information trying to find these answers. That is what research is all about, the more you look into something the more you discover, the more questions you find need an answer etc.
I have been looking mainly at Admiralty Pattern timepieces lately. I have been trying to locate what my AP3169 watches were used for, I know the answer now, it has only taken me about 5 years to find out.
In relation to RAF matters, I believe I have located a couple of new RAF timepieces, but will need to study my new information a bit more before I release the references.
In relation to the question posed by this thread, G.S markings on a MkV.
This means that this timepiece was rated as being for general service when it was last serviced. This can happen for a couple of reasons as far as I am aware.
The short answer is that it either did not meet the higher standard expected of this grade of timepiece or the specification and/or equipment requirement was obseleted, so the unit defaulted to the next level down.
I suspect for this example this happened in the early 20's.
This is the time when the RAF stores system and hence property marking requirements were changing. There were about 3 major changes over about 6 years in this period. It was not to about 1924 when things stabilised in this area. ( this is from the top of my head, I would need to do a bit of study to get the dates exact.)