The tech that almost didn't make it into Another Past

Every piece of technology Paul builds had to pass one test: could a very smart person with 50 years of foreknowledge actually build it in that decade, with that decade's tools and materials? A few of my favorites failed the test and got cut.

I'll share the ones that hurt the most to lose if there's interest — and I'm curious which of the surviving tech readers found hardest to believe. Nothing's off limits, spoilers tagged.