add &id_above=[highest observation id returned] or &id_below=[lowest observation id returned] to your URL to control which observations get pulled back. so for example, instead of using your 51st page query above, run the query for the 50th page, find the id of the last record on that page, then run the query again, resetting the page=1 and adding &id_below=[id of last record on 50th page].