This is, of course, exclusive to the Warhammer 40K universe that he came from.
The background for this pistol is quite simple. Verdiana(The planet and birthplace of Haakon.) was ruled by a series of major houses over the course of its history disconnected from the Imperium. The Lew family was one of those houses but during Haakons Grandfathers reign the house came to an end.
Another house had began to gain power and land and seeing it coming, had surrendered the lands in a bloodless takeover to protect his people. Though the house and many of its historical relics were destroyed as is typical fashion for a conquered domain, this pistol was given to his Grandfather as a gift for the respect it took to lay down arms instead of fight.
When the Imperium rediscovered the system and after a deliberation of almost 50 years regarding its systematic Exterminatus, the planet was spared, contact was made and the Purge War began soon after. At this point Haakon was already a young man and after the war was a well known hero. When the Imperial Guard raised regiments from the Planetary Defense Force and surviving troops, Lew was selected to command the 7th Rifle's, the first regiment raised of an entirely Verdianaian pool from top to bottom.
When he took command he took the pistol with him, however the Imperium was not entirely acceptable of the sidearms lack of Imperial imagry and branded the Imperial aquilas into the ancient wooden grips.


Below is another view of it, and a pair of pictures of a standard, line issue model.
http://i6.photobucket.com/albums/y2.....5at65746PM.png
http://i6.photobucket.com/albums/y2.....2at91251PM.png

http://i6.photobucket.com/albums/y2.....lf/M4112-1.png