Given that the oirginals were all rifled, it may well be that this is a replica that has been de-farbed, that is to say, all modern traces of manufacturing removed. This is, however, illegal in this country, I understand, although it is commonplace in the USA in the interests of HC. The eagle is a standard lock stamp for US government-issue arms of the period. If it is a repro you CAN still buy it as a Section 2 Firearm - a shotgun, but obviously only if you have the necessary authorisation.
If THIS -
https://www.freeads.co.uk/uk/buy-sel...w#.XdU4b1f7R3h - is it, then run away. It looks like an Indian-made smoothbore of modern manufacture that has been futzed around to make it look fancy. It is nothing like any Zouave replication I have ever seen and is, TBH, not even worth hanging on the wall. It might, in fact, be a non-firer, too, since he is selling it with no mention of the necessity to have a SGC.
The REAL Zouave is a fine-looking rifle and the example I get to shoot in the USA is equally as good as any Springfield.