They were still writing up BIBOP as the new hotness in 1977, so I'm pretty sure this predates BIBOP by a decent margin. But it's a fair point that the encoding

They were still writing up BIBOP as the new hotness in 1977, so I'm pretty sure this predates BIBOP by a decent margin. But it's a fair point that the encoding in PDP-6 LISP doesn't match what we'd think of as tagging today. They left the 0-pointer as a non-integer (probably that meant NIL), so the type test would have been done using two comparisons rather than a mask+compare.