If you want ALL the dope on these Mauser conversions, go to
www.gunboards.com and look for the "Commercial Arms" forum. There is a "sticky" at the top of the thread listing that has a collector's research on these guns.
Aside from the fact that it is a military long arm converted to a civilian shotgun, a "Zulu" is unrelated to the Geha/Remo/Hard-Hit-Heart guns--completely different critter.
These Mauser conversions are interesting, but a Mossberg bolt shotgun with a rifled barrel would do as well, I think. I suspect it isn't hard to find a shop that rifles shotgun barrels. One I know about is White Dog Gunsmithing in Ilion, NY (assuming they have survived the economic mess). And besides, then you'd have a genuine Mossberg!