MARBASECIDP will not give you any more information than CPSIDP. That variable predated the creation of CPSIDP and has been retained in the IPUMS database for continuity. The process that you described is the right approach for this, since as you mentioned the ASEC oversample all have CPSIDP=0 and cannot be linked to the March Basic sample (or any other samples). The observations in the basic CPS that do not link with ASEC in 2016-2018 were in the “split panel,” which was a survey experiment to test the 2014 income question redesign. You can read more about that on this thread: Discrepancies between number of obs in 2018 ASEC and March BMS and problem with matches in 2021 - #2 by Ivan_Strahof