I have understood from reading about magnet types from Tom Holmes and Dave Stephens that the composition of 50s alnico magnets is quite different from current alnico. Don't know when changes occurred but probably before Dry Zs were developed for Tom already experimented with magnet alloys in the 70s to get the right composition. Though it might very well be that Dry Z alnico composition is different from current magnets too. What I do know is that Dry Zs had original 50s enamel wire, which they ran out of in 1981. So maybe it's the wire (as Stephens also suggests).