1984 W126 300SD

Started to take out my guide rod mounts today to replace them. parts come in tomorrow. while i was loosening them the ones in the front came out fine.. however the arms on the captive nut on the other one was rusted through and spun/broke off. so i cannot get them out. i tried shimmying some various blocks to try to hold it down but no luck. the largest wrench i have is a 19mm and i can insert it into the hole (the closed end), but the "square" nut was just a tad bigger. does anyone have this part lying around and can find out what size closed end wrench fits? i think i will just go buy this one wrench and try to get it in there. and i have soaked and tried a lot of pb blaster before i attempted this job, but there was just too much rust on the arms of the nut to hold it together.

__________________
198? 300D ???K - Future DD
1989 300TE 285K - traded in '12
1984 300SD 186K - Sold in 08
1978 300SD 330K - Died in 05