The following numbers are from a 1985 P18 mixte,  As you can see there's a paper sticker and a stamped
number.  As noted earlier the stamped serial number is 8 digits.  The sticker breaks down as:

T=handlebar type
BU=unknown or burgundy
53=frame size in CM
This the only Peugeot I've see with this unique numbering.  The breakdown is as follows:

54=frame size in c

Date Codes

Component date codes are helpful in determining a models year.  Date codes are simply stampings on various
components which represent their date of manufacture.  Codes vary by manufacturer.  Older models may have
NO codes while newer models have many.  Codes can be found on cranks, hubs, rims, stems and other

A good source for interpreting component date codes is the Vintage Trek website.  

SR crank code 1982 PH12
AVA rim code? 1967? Peugeot
2 66  Feb. 1966 D. Alberts photo
Stem date code 1982 PH12
03 1982 March 1982
Hub date code 1982 PH12
14 82 14th week 1982
Serial Numbers

Prior to 1979 it is NOT possible to identify a Peugeots year of production from its
serial number.  Regardless of what you've read and where you've read it it's simply
NOT possible!!!  The number of digits
may indicate the decade of production with 5
digits representing a 50's model, 6 for a 60's model and 7 for a 70's model but this
has been proven to be false many times!!!.   Peugeot changed decal schemes very
frequently, changes in decals, along with component changes, make it easy to
narrow down a models year of production to within 2 or 3 years without a serial
number.  For bikes produced prior to 1979 decal schemes and component date
codes are the preferred methods of narrowing down a production year.
For bikes produced in the 50's, 60's and early seventies its more difficult to date
them because Peugeot didn't change decals or components often. Some models
were identical in every way for a period of several years.  For those models it takes
not only the serial number but a keen memory if you were the original owner or knew
the owner.

Some time around 1978-79 Peugeot labeled bikes with a small paper sticker.  This
sticker has the alphanumeric model, size, possibly a color code and a model specific
serial number.  I say model specific because these models also have a frame specific
number.  A frame specific number would have the following format:  Y012 12345678.  
At the time of this writing its not known what these numbers mean.  Y012 may be the
date code with '0' being the year and '12' being
the month the frame was built,
not the bike
.  '012' may also be '0' for the year with the second and third dgits
being a frame material code.  There are many frames with YX03 stickers and YX01
stickers which may be for Carbolite 103 tubing and Reynolds 501 respectively.  
Although no definitive documentation has been found/provided more than like;y the
numbers represent the year and month of production.  
Click Images