[N2IXK]

1. Since this is a 75 year old item, is there anything I need to do to prepare it for use i.e. install it on the TV and bring the unit up slowly on a dim bulb or my Sencore isolation transformer Variac?

Should be good to go right out of the box, as long as all the wiring looks good.

2. It has a "Series/Parallel" switch on it. I know about series install but what happens with a parallel install?

You set the switch according to whether the set has series or parallel wired filaments. If most of the tubes in the set have type numbers beginning with "6" and you have a big power transformer in the set, then you have a parallel setup. If there are a bunch of different numbers on the tubes, and no power transformer, it is likely a series filament set.

3. It's heavier than I expected. What's in this thing?

A small iron-core step-up transformer.

4. What modification can I do to the TV to achieve the same result as installing a brightener? Is there a way to put a potentiometer in this modification if I find I only want 0.5V boost or if I want to risk a bit more than the brightener would do?

You could maybe add a series resistor to cut down the amount of boost given. No easy to add additional boost other than adding a second brightener, which isn't recommended.