serial numbers - how does the pentax numbering scheme work ?

I have a number of A series lenses at home, so I decided it was high time I made an inventory. This has got me wondering does anyone know how the Pentax serial number scheme works ? Every A series lens I have starts with 5xxxxxx, except for my two 50mm f/1.7 lenses which have the series 15xxxxx and 23xxxxx.I cannot see an obvious pattern, but I am curious if it is possible to see when a particular lens, or camera body for that matter was made.

Cheers,

/Ian