Yes, the 10-rd mags from any Lee Metford (except the MkI) and SMLE will fit, but they won't be correct for this pattern. The 5-round commercial magazine is the right one. Just curious, how are you intending to use this rifle, for hunting, target shooting, plinking, or as a collector's piece?
Looking forward to seeing a photo of the other buttstock (I'm hoping it might be the original).