5 digit dunhill shape number?

[fishingandpipes]

Hadn't really looked before but just noticed that my lone Dunhill is stamped 51011. 5101 would make sense, a group 5 taper stem apple, but does anyone know why the fifth digit may have been added? Any potential story there beyond just being a mis-stamping?

 

[fishingandpipes]

According to this:
https://groups.google.com/forum/#!topic/alt.smokers.pipes/Fk-8EWxZXCs
5 digit codes were used between 1975 and 1978, but this is a 1983

 

[daveinlax]

I've been told the fifth digit is some sort of stock code. Over the years I have built a photo file of probably over 200 five digit pipes off eBay to try and find some sort of pattern. I don't have a clue. 8O

 

[7ach]

i have also spent some time trying to figure out what the last digit is all about. glad i'm not alone