Hi Stephen, there is room for preference I suppose, as some persons whom I happen to know have an altogether different approach to leather and how to voice it. There are stories out there that need sortin'! One of them is to use very hard sole-leather for pediatric shoes, and make long, parabolic cuts. This I have seen a lot in old Pleyels. These tend to have their noses broken off, in large numbers, given time. 
I use traditional oak-bark tanned leather, shoulders of cow. The first test is to bend it 180 degrees, stressing the outer layer. As soon as I see cracks developing while bending, I know I'm not going run the second test. Many sorts don't make it past 1. I leave more meat in the plectrum's shape, and my leather of preference is tough but not hard, which is why the other, er, .. faction disapprooves of it. But Frank Rutkowski gave it full green light! I have given some to Mercier-Ythier in Paris, who had a London-based Pleyel biggie over and his leather did not pass nr 1 when I tried it (with his expressed permission of course) So later he called me all in a panic and asked if he could have some of mine? 
Otherwise, I was under the suggestion that the indomitable Fred Sturm passed on the information and my source to you?   
Not too long ago I refurbished a William de Blaise biggie in Norway, having had all the jacks sent to me previously and reworked them in the shop. There are some leathery photo's in the photoalbum that you can acces through this link:
https://imageshack.us/a/cR64/1

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