As Sylvilagus said, a real No.5 receiver would have a lot of "unnecessary" metal removed by the lightening cuts. A replica Jungle Carbine made out of a cut up No4 should not have the same problem.
You probably won't have the "wandering zero," but if they left the full size Enfield sights on there instead of putting on the right rear sight for the carbine-length rifle, you may have to ignore the range markings on the rear sight while zeroing it in.