Thanks for the info music6000
Judging by Jon Patton's comments regarding dark/light resistance and dynamic range of the the VTL5C1 and my limited understanding of the finer points of data sheets (Help from better brains than mine very welcome) I'm not sure if the NSL-19M51 quite fits the bill for the Cardinal
from what I can gather it is especially finickey regarding the Vactrol/LDR part of the circuit
Definately good to know NSL-19M51 work well in the Magnavibe though, and I suspect will probably be good in plenty of other vactrol / LDR builds as well, seems that most circuits are thankfully a bit more forgiving than the Cardinal!
I've been using G5528 usually with diffused red 625nm LED (Shrink wrapped of course) in most of my own builds with good results so far, but always nice to hear of viable alternatives, especially if we can find them at reasonable prices. At the moment I'm considering getting some G5516, G5537, G5549 just to see how well any of them work in different circuits, and at a few pennies each it's not much of a gamble if they aren't a success, could eventually be perfect for something though.......hmmm that'll be another box of components