After reaching cruise altitude; an ERJ-170 Captain was asked what were the 'knobs' sticking out of the cabin Passenger Convenience Units (PCU)'s. Upon landing; maintenance informed crew; the 'knobs' were locks; preventing oxygen masks from dropping.

Narrative

Crew was allowed down to aircraft approximately 35 minutes to departure time. Preflight duties were started and Flight Attendants noticed that there was no catering on aircraft. They both went to aircraft one gate over to grab some 'essentials' for our flight. Upon returning boarding started. During the boarding process the 'B' Flight Attendant came up front and stated that all the personal breathing equipment (PBEs) in back were missing the lock. It was approximately 5 minutes to departure so I called Maintenance and they said we could place liquor locks on the PBEs and Maintenance would come out in ZZZ to put a more permanent lock on them. We then departed from the gate. After reaching cruise flight; the 'B' Flight Attendant called back up and asked what the 'knobs' sticking out of the PCUs were. Not knowing what he was talking about; I thought it was just the tabs used to keep the PCUs up. Upon landing and after maintenance boarded the aircraft; we were told that those were in fact locks to keep the PCUs up and the oxygen masks from dropping. This was the first revenue flight for this aircraft after a heavy check. After a heavy check; more time is needed and a better check of the aircraft needs to be accomplished prior to returning to revenue flight.