I think the simplest way to use a computer in the pre-PC era to profit would be to apply the computational power to calculations that nobody else could afford t

I think the simplest way to use a computer in the pre-PC era to profit would be to apply the computational power to calculations that nobody else could afford to make - so things like stock trading could be very exploitable, as you would be applying arbitrage methods that the market literally could not keep up with. If your intent is to accelerate research, on the other hand, obvious candidates would be CAD systems and simulations. A lot of lighter/cheaper machine designs have come from finding better minima of cost/weight/strength, and microprocessors themselves are beneficiaries of CAD technology. If you're in the pre-industrial era, being able to transport back blueprints of modern designs would still be of benefit, even if the economy isn't ready for all of them.