Managed to grab a quick look. Of course, as mentioned above, how can we always be certain something wasn't switched over many years ago, especially if carried out by an armourer, or a civvy who knew what he was doing? But, as far as I can judge, none of these, apart from the shiny gunmetal butt plate on the left, has been replaced. Note four of the six are mazak, including one on a butt from a 41 Maltby.
I also checked a few of my rifles that came to hand; a 41 Maltby, a 41 BSA, a couple of 44 BSA's & a 45 BSA. One of the 44's & the 41 BSA bore gunmetal butt plates, but as I fitted them that doesn't tell us much. The other rifles all bear mazak butt plates.
Having said that, the butt at extreme right shows why there was a tendency to replace them with gunmetal or steel, as the mazak/zamak goes brittle & cracks with age............so in the post war refurbishments there may even have been a deliberate policy of replacing mazak....I don't know, but it would make sense. If Peter is inclined to chip in he may be able to shed some light on the issue.