It's not an overprint, but rather a postmark . Not sure what it stands for, but here is an example, on cover, in an old Siegel auction.
http://www.siegelauctions.com/lot_g.../2020&symbol[]=All&lotclass=All&syear=All&pfoper=All&pseoper=All&pfgrade=&psegrade=&gandor=or&keyword=&catselect=eq&pscolumn=default&pssortby=&sortord=DESC&photo=&calledfrom=lkp