Original: By consulting the colophons in the Left Ginza, Jorunn J. Buckley has identified an uninterrupted chain of copyists to the late 2nd or early 3rd c. AD.
Simplified: Jorunn J. Buckley looked at these special marks the copyists left in the Left Ginza. That way he was able to show that the copying of that text goes back to the to the late 2nd or early 3rd century AD.