You and any big company patent litigation department.
I was in Google's. The thing about old manuals and technology is:
It's all prior art. All of it. It doesn't matter at all if it no one used it and it faded into obscurity (I see you there: OS/2, BeOS, and Magic Cap). An inventor is assumed to be familiar with every single piece of technology that was published anywhere, anytime. You build a case that some patent was anticipated or rendered obvious by X, and you've usually won.
Working hardware is useful, but not essential.
If someone sues you for something you're sure was done in 1980: that 1980 manual is gold.
I was in Google's. The thing about old manuals and technology is:
It's all prior art. All of it. It doesn't matter at all if it no one used it and it faded into obscurity (I see you there: OS/2, BeOS, and Magic Cap). An inventor is assumed to be familiar with every single piece of technology that was published anywhere, anytime. You build a case that some patent was anticipated or rendered obvious by X, and you've usually won.
Working hardware is useful, but not essential.
If someone sues you for something you're sure was done in 1980: that 1980 manual is gold.