Back in the sixties, calculating spoke length by formula wasn't practical. There was no internet yet, and hand held scientific calculators were pricey toys that only engineering student could afford. The rest of us were left to struggle with pencil/paper, slide rules and sine tables.

Builders kept notes, so they never had to calculate spokes for the same wheel twice. I used a simple calculation method based on the distance of holes from the centerline, the rim OD, and the drop to the top of the nipple. Then added correction (fudge) factors for the flange separation or CTF for rears, and could generally hit my target within 1mm either way. I still use this method because it's simple and reliable.

As Ric noted in his blog, spoke size availability was thin, and back then there was no UPS, FEDEX, overnight shipping. You had to place large orders with local vendors who delivered once a week, or combine with bike orders for truck freight. Parts orders from non local vendors went Railway Express, or Greyhound and had to be picked up at the terminal.

The logistics alone, quickly trained bike mechanics in how to make do with what you had.