Does anyone know if IVSFX (class A shares) is the correct symbol for Investor AB ?

Generally speaking you'd be buying the B shares.
The A shares are the ones with higher voting rights that exist for the founding family, and they aren't as liquid. Both have the same economic interest.

It's much like same answer for Berkshire A and B shares, but without the big difference in share price.

I can't really comment on the US pink sheets, as I am interested in the shares listed in Stockholm, INVE-B.ST
But I think the one you'd be looking at is IVSBF.

As for liquidity: don't worry about low volume per se. (same with any odd ETF or secondary listing). Rather, check the bid/ask gap during the few hours that both US and European markets are open. If the bid/ask gap is small, it doesn't matter much what the volume is. The arbitrageurs might be making sure you're getting a good deal.

Jim