While this is off topic, I use my '53 Nash for daily transportation and for Model T parts hunting. It has a 6-volt system and a generator with voltage regulator.
In my parts searching I found a Sun Volts/Ampere tester, Model CB. I'm looking for a manual, reprint manual or photocopies of how this thing works. I'd like to use it on the Nash.
I have one of the Sun Master Motor Testers and this style of VAT (Volt/Ampere Tester) had originally come with it and is covered in the manual. It details it's usage and hook-up, and covers some specifications as well. Tomorrow, I can scan what I have and send it to you in an e-mail.
The Master Motor Tester came with the Battery/Starter Tester, Volt/Ampere Tester, Timing Light, Compression Tester and Cylinder Balance Tester and they were stored in the cabinet under the machine (along with the steel case full of spec. cards). Originally, if you hadn't bought the MMT, you could have bought just the individual testers and they would have come with their own metal storage cases and manuals.
I know that there are a couple of different "original" versions of operators manuals for this listed on eBay right now (reasonable prices, Buy-It-Now). They are for earlier models than the one that you have shown, but the hook-up and operation are the same. The big knob in the center is attached to a carbon ring that is used to increase/decrease the resistance, a lot of times the ring is cracked or broken, so you may want to pull the front face off and check. I will send you a Private Message tomorrow when I get the manual pages scanned in.