Well, IC3A and IC3B are just functioning as inverters ...

The thing with the bubbles is probably an old technique that we used to use called "pushing circles".

You can shove all the circles on one side of a logic gate to the other side of the logic gate by exchanging the gate type (AND->OR, OR->AND).

And then you can ignore lines with two bubbles in series (ie. a double inversion).

There are a bunch of tricks that we used to use when you had to do everything on graph paper. It's actually a good thing that they are lost to the mists of time as it means we have progressed enough that they don't matter anymore.