There may have been a reason to use a part like that, but in history, that the condition they expected it to be possibly put in, nowadays, no longer exists...
Here's another way to look at error is schematics and their ways of getting people hurt, if not electrocuted.
Above is a 138XLR...
However, better clarity...
The Above is a Cobra 135 XLR - it too, uses a typical "Spike reverse polarity" diode but notice the parts and how they are connected AROUND it...
Again, a closer look...
The radios - to obtain an UL rating, had to survive certain conditions - including what was called a Hi-Pot test...
Hi Pot means Hi Potentials. They didn't want operators getting killed when they decided to put up antennas, or use bad house wiring or poorly grounded system - from feeding back into ground thru the best lowest ohmic route possible - Bags of Water - often called Humans - that can turn into a great carbon trial just for this purpose.
Your 138 doesn't show this ,but that era, it shows in the level of Hi-Pot protection - so they could at least list on the schematic that they have placed a hi-voltage device in an area to protect from high-voltage and the user from becoming a Ground rod from it...
Even the 139 BASE shows this part...
The 139 BASE radio from Cobra - proves this "idea" - because they are possible to be operated in specific conditions of Positive Ground ignition systems - at least the Mobiles could.
When you "mis wire" a CB to a Positive Ground ignition system, the CASE has 12 volts on it, it's not grounded it live. IT's wired backwards, so the protection diode is designed to pop the fuse - and if a high voltage was on it, the typical 1N4007 Diode may not survive the biasing condition (PIV rating) to keep blowing the fuses until the condition was cleared - hence the reason for that voltage retied part...
Same for Base radios using insulated chassis system...
The Ground at the antenna - versus the potential at the radio - can exceed the 12 volts these radios can run on. When they are insulated chassis - you may or may not realize that if you touch the unit and your seated in say a chair, but the table is metal - your arm can become a conductor to ground the radio to the table. Can be painful, if not lethal. Only works if the Table is grounded or touching a wall plate that is grounded.
It may sound laughable now, but back then UL labs was pretty strict on making radios and other Electronics, safer than previous generations of products that wasn't safe, because enough was not known earlier - to prevent the tragedies - which brings us to today, being that we will never see these dangers practiced as long as we follow directions.
It's the "Follow Directions" part, that scares the h*ll out of me...
