Frankly, you shouldn't be using any knife in such a way that it would close up on your fingers, no matter how strong the backspring was or wasn't. As far as weak springs, there have been zillions of super-cheapo "Barlow" knives produced throughout the last century. Really cheezy knives, with stamped sheet metal handles and bolsters, etc. Pretty crappy all the way around.
The familiar "bag shape" handle seems to have come about in the mid 19th century. Before then, the handles had a "ball butt" sort of shape. Bolsters are T H I N and forged integrally with the liners. Handle scales would be horn, antler, or bone. Nail nicks seem to have begun to be put in the blades in the 1790's. Prior to that, the blades were broad, and you pinched them with your fingers to open them (better than a nail nick anyway). Unfortunately, NO ONE makes anything even remotely resembling an 18th century (or even a 19th century) Barlow knife today.
These photos are stolen from the interwebs, and are NOT mine.
These above are VERY typical 18th century Barlow knives. The remains of knives of this general form are found in large numbers in colonial American archaeological sites.
This is an early 19th century Barlow knife, with a nail nick.
From "Joseph Smith's Key to Manufactories of Sheffield", 1816
I do not know the date of this two-blade Barlow knife. It's obviously old, and still has thin forged bolsters. I'm assuming maybe mid 19th century.