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These Super Famicom cartridge PCBs have what appear to be uniformly-spaced traces extending all the way to the extreme left and top of the board. They don't look castellated (and there are easier ways to make test points besides). The ICs are traditionally mask ROMs, so there shouldn't be any additional interface required for programming.

PCBs showing strange traces near the edges

I would guess they were going for some sort of isolation between the various fills, but that wouldn't explain why they needed to be uniformly spaced at the edges. The source of the edge traces seem to be "non-essential" in that they don't run to any of the "important" chips.

To my eye, they're just antennas without a purpose. A friend suggested the boards might have been machined on the left side (removing the mouse bites) after-the-fact, making this even more mysterious. Can anyone shed some light on what purpose these might serve? Thanks!

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They are almost certainly part of a panel test fixture. Certain traces of each board on a manufacturing panel are brought out probably to a header on the outside edge where a tool is connected to perform functional testing of the boards. Once the boards have been tested they are de-panelized, but the traces remain and are never used again.

Here is an example from Dave Jones at the EEVblog: https://youtu.be/UESc7ms4efo?t=132

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    \$\begingroup\$ There are reasons why this is not that commonly used today, specifically : MFGs will abhor routing copper 2 exposed copper at the edges is actually not that desirable. 3. if the traces are closer spaced they can short together as the bit moves through the material, which can be a problem 4. They are indeed antennas and can impact the circuit \$\endgroup\$
    – crasic
    Jun 24 '20 at 1:40

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