I think that the "2103" stamping is problematic.
The font seems off, and it looks like the stamp was made by a machine press, leaving marks from the stamp/tool itself outside the "2103". As far as I can tell the "2103" shape corresponds to a Group 2 billiard with a fishtail stem. This is all well and good, but my Google-fu is only bringing back pictures of a slightly thicker and longer shanked pipe than the one you are showing us in pictures here appears to be.
http://dunhillpipes.com/products/copy-of-dunhill-bruyere-group-2-pipe-2102a
http://www.smoke-king.co.uk/acatalog/White-Spot-Pipe-Bruyere-2103---Group-2-Billiard---by-Alfred-Dunhill-5227.html
http://www.iwanries.com/Dunhill-Bruyere-2103-P13647C168.cfm
This all being said, the pictures you've posted don't give us great angles to check out the pipe. Do you have other photos?
I'm not a Dunhill expert, but there are others here who are. More photos will help with authentication.
Regardless, I can see why you are questioning the pipe's authenticity.
-- Pat