Here is what I believe to be perhaps the rarest combination of parts put together at the factory that you could ever imagine. I have looked this over real close and I believe it to be all original. Read on to see why I think this is so special.
First of all, I bought this at the Michigan Antique Radio Club Extravaganza in 2006 and thought it was unique because of the battery door cover. It has the flared bottom seen only on Royal 300’s. So I figured that maybe someone swapped a maroon 300’s door and put it on the 500.
The front view shows the knobs of a second generation 500B, but the pointer on the plastic dial indicator is black, which indicates an early 500B. A few of these early 500B’s with black lines have handwired chassis even though the knobs would indicate a printed circuit board. But did this one did have a hand wired metal chassis? Yes! Yeah!
 
The label on the battery door was not for a Royal 300 like I thought, it said the chassis was a ……. what? Can this be right?
Sure enough, there were 7 beautiful blue Raytheon transistors looking at me as this set was equiped with the rare 7XT40Z chassis. No one knows for sure just how many of these handwired Raytheon chassis were made, but based on combined knowledge from other collectors, auctions and conversations, if it was any more than 1-2% of the 100,000 handwired units, I would be surprised. 
And how many hybrids were made? A hybrid was made during the transition from handwired boards to printed circuit boards. Apparently the supply of all the parts did not balance during this time, so some mixed models were made. But another strange part of this radio is that the serial # is 33,684 which would indicate that it was laying around for several months. The other hybrids that have been reported have had serial #’s ranging from 90,000 to 121,000. Very strange.
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The final comment I have to make about this unique radio is that it appears to be all original and it plays like a champ. Has anyone else encountered anything like this before? I would be very interested to hear about it. Gary
Wow…..those blue Raytheons really jump out at you. That is certainly a radio full of mysteries. The battery cover is definitely for the Royal 300, but the 300 label should read 7AT42Z. The date code on the tuning cap is 001 (first week of 1960) and must be a replacement. Can you give us the date code from the output transformer? The early (for a hybrid) serial number may well mean that this was pulled from stock and later became a prototype for the “B” models. Years ago I saw a 500 with the early volume knob and pot but the later tuning knob and vernier tuning capacitor and thought it was only a ‘cob job’. I hope it turns up again since it may also be an early prototype unit from Zenith. Thanks again for these photos. Collecting the early Zenith 500’s has just taken a new dimension.
The output transformer date code is 618 or early May 1956 which would be right around the trasition time.
Hi Bill,
long time ago i search for Zenith Transistor Radios Royal 500.
You find informations to this Zenith under
http://www.radiomuseum.org/r/zenith_royal_500_7zt40z_3rd_production.html
i think this one is really rare.
It would be verxy helpful for me you can send me the images from that model to my E-Mail adress.
You know we give you credit for the images.
Long time ago i bouht one Royal 500 in white from you and iam very proud about the radio in very good shape.
Kindest Regards and stay cool,
Hans