Images from playvectrex.com (thanks Brett), originally from F.Bartsch. 
Some images have been manipulated by me to make the screen show up better.

From an ex-GCE employee :
"Here is a heads up on a future eBay auction. I have a VECTREX Cartridge Tester that is the last of its kind. There were only a few made and all the others were in Hong Kong and long since deceased. There were maybe 10 made and the cost was about $3,000 to $5,000 each to make. The auction will not be for a few weeks but I thought I would let you know in advance. It is very cool and the last of its kind. I doubt that anyone has heard of them. It is made from a vectrex and has a custom ROM in it. It reads cartridge check-sums. It was used in Hong Kong as the final test on all cartridges before they are boxed and shipped. I was looking forward to posting it on eBay just to advertise that it exists. If you still want it, let me know and we can work something out.Every Hong Kong made cartridge you have was tested on one. It is a Vectrex. It is modified to read checksums. You put in the Cartridge. Turn it on. Push a Button and it reads the check-sum. Then next cartridge. It will have a $500 starting price on eBay. It is "the rarest Vectrex ever"

Editor's note : I'd say the color prototype is a fair bit rarer still :)

From Frank Bartsch :
"I´m selling my collection of old video games and old home computers. Here I´m selling a Vectrex main unit, which has been modified to a very special one for testing purposes: THE GCE CARTRIDGE TESTER It looks like a common console, but after switched on, you can see it´s speciality on the screen. This console comes from USA and out of old GCE equipment. GCE is the company, that sold the Vectrex in the 80s. This unit has been built for testing the game cartridges. This is a colletors must-have I think. I think that there will not be more of them around in our Vectrex-community. ATTENTION: I´m selling this unit WITHOUT the controller (as any Vectrex-collector has one) and without a game cartridge. But I will add a power converter for 220V in Europe.."

The serial # is 106320 A Made in Taiwan

The screen says:
"GCE CARTRIDGE TESTER"
"HOW LARGE IS CARTRIDGE ?"
"BUTTON #1 = 2K"
"BUTTON #2 = 4K"
"BUTTON #3 = 8K"
"PRESSING BUTTON STARTS TEST"

So I plugged some carts in for testing.
Here are some results:
BEDLAM (4K) is FDB3
SPACE WARS (4K) is 991C
RIP OFF (4K) is 2087

There are no sounds all in this little program. The Button 4 has NO function at any time. The joystick even has NO function at any time. When you plugged in a cart for testing and have started the test, the next little screen appears:

4 K CARTRIDGE
FORMING CHECKSUM

And a little bar which shows the working progress (from left to right) is ON the words "FORMING CHECKSUM".

When done, the next screen appears, which is the same like the first (now only with the checksum-result at the bottom).

From Chris Romero :
"From what I know of this unit the only difference between the cart tester and a regular Vectrex is the ROM. The ROM is a specially built pieces of software that does only one thing. Checks carts."