I've been spending some time with some old PCBs from the 70s. I have seen this pattern on some, which are these short traces that appear as little whiskers in between each of the IC lead/leg through-holes. (This pic is obviously unpopulated.) They aren't connected to anything and are not soldered to anything when the boards are seen populated. What is their purpose? Is this a soldering aid of some kind?
That is a good question.
Olden days (for you :)), we called the PCB design process Artwork. In order to make a "film" for the photo etching process of copper clad PCB, people had to "draw" the film. Actually a film was "developed" from something drawn on a mylar film usually, using drafting pens or using black tapes and ready made various foot prints.
Long story short, the taping required precision :-) when the traces have to pass between IC pins. So, it was convenient to have footprints with the traces already in the middle of pins, with good clearance.
They're part of the layout template,
70's was before computerised layout, track layouts were done on paper and then photographically transferred to the circuit board.
If there's a need to run a trace between two pads they offer connection points for that.
pre-patterend sticky tape was used for the part outlines, and fine crepe or vinyl tape was used to lay out the traces. (source: Altium)
those dogbone traces were called solder traps. Before full automation, soldering was done by hand. It was very easy to flow too much solder onto a joint. They prevented solder runoff onto adjacent pins which would in all likelyhood short out the chips.
I ruined alot of 7476 Flip Flop IC chips back in the day ;-)