He (Lawson) can only point to his paper from 1967 and the fact that in 1964 he was asked to join PL/I team due to his earlier published works on linked lists.

https://dl.acm.org/doi/epdf/10.1145/363332.363344

PS He didn't mention it in his recollections [1], but 1978 paper The Early History and Characteristics of PL/I [2] claims that a paper was produced in October 1965.

[1] https://archive.computerhistory.org/resources/access/text/20... [2] https://dl.acm.org/doi/pdf/10.1145/960118.808389