The Alpaca wool found in original jackets such as the B-10, B-15 and N-1 is very soft to the touch and can be worn next to the skin with no irritation, however the stuff found in BR jackets is some wool/synthetic blend that looks a bit like Alpaca but is much harsher on the skin and not half as warm. I can't comment on the other makers.
Alpaca drives me round the twist, and I can wear any other wool against the skin.
I really think the alpaca thing is an entirely subjective thing, some seem to have not a jot of a problem with it, others, it drives them up the wall. Peter might have a point about how it's been processed/spun, although I personally can't tolerate the stuff and I even bought the softest alpaca scarf I could find when we lived in Chile, but even after a couple of minutes I'm scratching like mad.
Best try yourself out on the stuff first would be my advice.
Jackets like this weren't meant to be worn next to the skin. There would have been underwear, uniform, and the B-10/A-9 jacket and trousers had the F-3 heated flying suit underneath as well. Not many blokes wore just a t-shirt under their B-10 at 30,000 feet! A bit excessive for trip to the shops, I grant you.
I wear a ELC B10 with just a T shirt and to date have never had a problem or case of itching, however some of the very cheaper versions I have heard people complaining about the "wool" used does cause itching.