a.lber.to
2357
In a nutshell...
...the last digit of the Ref. numbers indicates whether the watch has a solid back (0) or whether it has a display back (7).
As per the serial numbers, these are assigned to the watches as they are made, across models/references. Hence you may have watch number 3452 L which is a time only watch, and watch 3453 L which is a tourbillon. Each serial number is shown in an abbreviated form on the dial of the watch (just the three or four digits, as each run goes from about 500 to 5500), and repeated on the back of the watch, followed by a letter. This means that you may find two watches which both have the same number of the dial, however when you look at their backs you will see that the same number is in each case followed by a different letter (i.e. 3452 L and 3452 K).
I hope this helps clarify things a bit... 
In a nutshell...
By: a.lber.to : May 7th, 2005-09:09
...the last digit of the Ref. numbers indicates whether the watch has a solid back (0) or whether it has a display back (7). As per the serial numbers, these are assigned to the watches as they are made, across models/references. Hence you may have watch ...
Dating a Breguet
By: barnaby416 : May 7th, 2005-07:19
Thank your Alberto for your reply. If the serial number is assigned in sequence as the watch is being completed, then it should be possible to date a Breguet. Is the factory the only source of information, or are there unauthorized books or websites out t...
Ahhh...
By: a.lber.to : May 8th, 2005-01:01
I did not realise you were referring to a Type XX watch - I should have read your post more carefully. As far as I know, all the watches in the Type XX/XXI families follow a different, independent numbering schema, and these make up the bulk of Breguet's ...