I don't go back as far as the '50's (with reference to uke's) but I've got a couple of "
Jumping Cow" felt picks, one "proper" felt, the other "artificial" felt, that are both quite soft, much like those I remember from the late '60's/early '70's. I prefer the artificial one, but I can't really say why
they both sound much the same on the strings, but the artificial feels nicer to hold, IMHO.
However, I do also have some Mahalo-branded felt picks that feel truly horribly hard, but again, sound virtually identical on the strings as the other two.
I've not used any of them sufficiently to be able to comment on long term wear and I'd happily use any of them to get that "felt pick" sound if I needed to, but if/when I need to buy more, it'll probably be a Jumping Cow artificial I'd look for first
Hope this helps ... as always, YMMV