Secondary reading marks occur in various places throughout the Chester Beatty Biblical Papyri. There are a couple previously in Hebrews, but they kick in quite consistently at 2.5 (see the picture below). In general they serve to mark the thought units within the OT citation (Ps 8.5-7) - it is less clear to me what is going on in v5 (as if it were me I would place a mark after MELLOUSAN to separate the prepositional clause).
One interesting feature of P46 here is the reading TIS in v6 (it is masc. suggesting a personal question: 'who?', as opposed to TI which is neut., 'what?') [which, of course could be original to Hebrews since it is not assimilated to the LXX, which in most witnesses has TI]
Anyway Zuntz had an ingenious theory that v6 in P46 (reading TIS) consisted of a question and an answer: 'Who is the man whom Thou mindest?' 'Truly the Son of Man, for him Thou visitest.' Among the various objections to this (from style, theology, syntax), perhaps we could make two objections from P46 itself: firstly, whoever put in the reading marks surely did not recognise the two clauses as question and answer since then we would expect an extra reading mark (as it is the marks seem to treat the double clause as a unit, suggesting parallelism as normally taken); and secondly, that if P46 had wanted to indicate that 'Man' and 'Son of Man' were christological titles it could have used nomina sacra for ANQRWPOS.