try pulling the box at the back as per usual
BUT
pull the trigger at same time - it should allow the box a smidge more wiggle room forward
then eventually pop up at the spring guide area
it has to go unless somebody cast the damn receiver around the box
nah pull trigger and it should give that fraction more wiggle
perhaps when refitting a tiny file by top of receiver/spring area to get her back in
looks like one of them QD spring boxes from say TomTac or HelmetWorld (snigger)
did it have a micro switch trigger ???
reckon best bet is pull trigger, keep front of box down and with a large screwdriver
lever at the rear just under spring guide area, sort of forward & upwards slightly