These boards were basically laid out using masking tape on transparencies. The rounding comes naturally from turning a corner with the narrow tape. The teardrop

These boards were basically laid out using masking tape on transparencies. The rounding comes naturally from turning a corner with the narrow tape. The teardrops around pads and other more complex shapes (like footprints) came from pre-fabricated stickers. The layouts were produced in 4:1 or similar scale and were optically reduced during photolithography of the resist on the actual PCB (just like semiconductor masks). Way back then you also had some concerns about manufacturability because sharp corners tend to be undercut during etching when your process is poor. That's probably why they usually added little fillets to T and similar connections.