All matching can be a good indication a gun is all original but it can also be miss leading.
With just a little study you can tell a DWM font from a Mauser or Simson or HK part, so the mixing of manufacture parts isn't a real problem. For example an Imperial DWM "3" will usually have a rounded top and one from Mauser a flat one. The sear bar on a DWM will be SN stamped on the upper step and one from Mauser on the lower step.
Finding a correct part both in manufacture, SN and time is a difficult thing to do. The odds are against you, yes is does happen but with very low frequency. In cases where you suspect a parts gun of original parts you then need to look at the overall condition of the individual parts. If a trigger retains 90% straw and the other straw parts only 40%, you can be pretty sure someone got lucky and found that rare part they needed.
It doesn't happen very often at least not to me
Vern