Audemars Piguet

Ancienne Le Brassus
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Good question- no real answer :)

Thanks for taking the time to reply to my post.

The question you have is a good one and I've tried to answer it for myself a while ago and didn't come up with too much.
Maybe a fresh pair of eyes to the following info will help?

Here's what I've seen.

AP used letters after serial numbers only on wristwatch movements. I've never come across this on a pocket watch movement.

They used the letters 'C' and 'D' after serial numbers as early as the 1910's only on woman's wristwatch movements:
some examples are:
19906D 1917
26723C 1920
And I've seen some from ca. 1912 but I did not note the serial number at the time.

As to the use of B on wristwatch movements for man's watches. I've seen this start happening only in 1942:

Here's what I've observed:
Man's VZSS/SC/AZ line of cals. (I.e. seconds at 6', seconds at '9 for the 'world' timer models, and center seconds)
45885B 1945 VZSC
46582B 1943 VZAS
46604B 1943 VZAS
47509B 1943 VZSC
47732B 1944 VZSS
So for these, I have not observed VZ movements from within 1942-1944 that do not have the B next to the serial number. After 1944 it seems that
this system of putting a B stopped. As the movements after this do not have the 'B' (Also another difference is that the writing on the movement is not
engraved by hand anymore, but rather made by machine).

Man's square cal. 10" TS
46762B 10"TS 1945
Ca. 10"TS without "B"
47817 10"TS 1944
47748 10"TS 1944
Here we do not see any consistence, and the B appears into 1945.

Man's 9/10" RS Cal.
46886B 9/10" RS 1943
47572B 9/10" RS 1944

This same 9/10" RS cal without B
45541 9/10" RS 1941
48648 9/10" RS 1945
The same goes here, although my data for 1943-1944 is scant for this movement number, it might very well be the same as the VZ's where they were all given a 'B' after the number.

And yet this info is contradictory when it comes to Chronographs:
In 1942-1943 I don't believe AP did any 'calendar' only watches, but rather chrono-calendar combination.
Here's what I see
A chronograph-Date watch with 12 hour counter, moonphases sold by cartier in the 1950s, but manufactured in 1943
Is 46936B VZ cal.
Yet most other chronographs did not have the "B" after the serial:
43793 VZ cal 1938 simple chrono
45980 VZ ca. 1942 Chrono, Date-Moon phases
46842 VZ cal. 1943 chrono-12 hour counter

But as I said, on the few pocket watches from 1942-1945 I've seen, they do not carry a "B" after the serial number nor any other letter.

So as you can see, there does not appear to be a very clear reason (for me at least) as to the purpose of the "B".
One last note is that the export marks of ROR or those for B Mayer on the balance bridge do not correspond in any way that I can see
to the appearance of the 'B'. I.e. it is not related to whether the movement was meant for the Euro zone, or for the US....

So if you can glean any info from this, I'd be more than glad to hear it.
In my opinion it's a naming convention that they seem to have started at the same time that AP started using the VZ ebauches for regular 'time only' models which was
in 1942 (they were using them only for regular chronographs in the 1930s). And if i remember correctly from the Brunner/Pfeiffer/Wehrli book (where most of the above serial no's appear) the use of Valjoux ebauches for 'time only' watches was somehow related to the economic difficulties they had during the late 30s, but I might not have this story completely straight I remember reading in an old auction catalog that a letter from AP accompanied one of the VZAS 'world time' models and it stated that AP started making these models from spare chronograph movements, and that ca. 100 'world time' models were made in total between 1942-1943.

Hope this helps, and if anybody knows, I'd like to hear from them,

Have a good weekend,
Sebastian