Was knob and tube the only wiring used back in 1900?? Or was there another type of two wire system?
do you mean was there another system without a ground. if so yes. If you mean was there another system that ran 2 separate wires . I do not think so.
Wiring in open construction (attics, crawlspaces and in the wall and ceiling cavities) in the early 1900’s was required by the NEC to be installed via the porcelain or glass knobs and tubes. Twin wires (two conductors wrapped together) was not allowed unless protected by conduit, moulding or armor.
Also, the use of staples was strictly prohibited.